When i.e. artillery wiped out a whole file of a company, the formation would close up from the sides to eliminate the gap. This would increase the size of the intervals between companies in the battalion, so in a three rank formation, this process only continued until the formation had a certain number of files, whereupon it would transition to a two rank formation.

In terms of reserves, the Napoleonic Wars witnessed the transition from the classic battle of lines to what I call the 'battle of reserves'. While 1700 century armies usually tried to form a solid wall of men, the Napoleonic battle line was highly fragmented. While a more or less continuous firing line of skirmishers usually formed the front, the main strength was usually formed in two lines of battalion columns, each with a distance equal to its frontage in line between it and its neighbors, called a deployment interval, the second line being roughly a musket shot behind the first and offset to stand behind the first line's intervals. The corps, divisions, and brigades making their attacks did so without much regard for alignment with their neighbors, and even individual battalions could scout out their own path and angle of advance, subdividing to attack in multiple waves.

Over the course of a major battle, the battalions in the front line would get blasted into cinders as a matter of course, whether they were winning their engagements or not. Moreover, as every engagement is followed by a critical phase of disorganization, confusion, and weakness in the victors as much as the vanquished, reserves committed at the right moment often had a resounding impact. *

As a result, Napoleonic armies were very deeply echeloned. The Battle of Leipzig is a good example. Napoleon had three army corps and one cavalry corps in his first echelon: each of the army corps would have fought in two lines, with more out ahead as skirmishers. Behind them, he had two more army corps, three cavalry corps, two Young Guard corps, one corps of Old Guard, and the Guard Cavalry Corps. That's four in the first echelon, and nine in the second, third, and fourth echelons. Similarly, the Duke of Wellington fought the battle of Waterloo with 30 battalions in his first line, 13 in his second, 60 squadrons of cavalry in the third and fourth, and another 70 squadrons and battalions further to the rear. Only about one-sixth of his tactical units were in the first line.

As a result, even the complete crushing of a battalion in the first line by fire or cavalry would have only a minor effect on the course of the battle, there being ample reserves to counterattack the victorious enemy and restore the situation.

*I'm just going to paste in a bunch of quotes I have saved stressing the importance and role of reserves in Napoleonic and later armies.

Langeron was a French emigre who commanded a Russian corps during the Battle of Leipzig:

I believed the position was assured, and went forward of the village to establish a chain of outposts. At this moment Ney [...] launched against me so unexpected an attack, and so impetuous and well directed, that I was unable to withstand it. Five columns, advancing at the charge and with fixed bayonets, rushed at the village and at my troops who were still scattered and whom I was trying to re-form. They were overthrown and forced to retire in a hurry. I was swept along by the fugitives, but I really cannot blame their sudden retreat because it was impossible to hold out, and I must confess that they moved as fast as I could manage .... Fortunately I still had considerable reserves, and after letting the regiments which had been expelled from Schönefeld pass through the gaps between them, I soon did to the enemy what he had done to me, because my columns were in good order and his troops were by this time scattered.

Carl von Clausewitz was a Prussian officer who fought in several major battles and dozens of minor ones, and wrote the great treatise On War:

When [war] consists of a lengthy interaction of mutually destructive forces, the successive employment of force certainly becomes feasible. That is the case in tactics, primarily because tactics are chiefly based on fire power; and there are other reasons as well.

If in a fire-fight a thousand men face five hundred, the sum of their losses may be calculated from the total forces involved on both sides. A thousand men fire twice as many rounds as five hundred, but of the thousand, more will be hit than of the five hundred, for it must be assumed that the thousand will be deployed more closely. If we suppose that they suffer twice as many hits, the losses on each side would be equal. The five hundred, for example, would suffer two hundred casualties, as would the thousand. Now, if the force of five hundred had kept an equal number of men in reserve, out of range, eight hundred able-bodied men would be available to each opponent.

But on one side five hundred men would be fresh and fully supplied with ammunition, while all of the eight hundred facing them would be to some extent disorganized, tired and short of ammunition. To be sure, it is not correct to assume that because of their greater number the thousand would lose twice as many men as the five hundred would have lost in their place. The greater loss sustained by the side that held half of its strength in reserve must be counted as a disadvantage.

It must also be admitted that as a general rule the thousand may initially have an opportunity of driving the enemy from his position and forcing him to withdraw. Whether these two advantages balance the disadvantage of opposing, with eight hundred somewhat battle-weary men, an enemy not appreciably weaker and who has five hundred completely fresh men, cannot be decided by further analysis. We must rely on experience; and few officers who have seen action would not grant superiority to the side with fresh troops.

[…]

We have explicitly stated more than once that as a rule the final outcome turns on the ratio of unused reserves still available. A commander who recognizes his enemy's distinct superiority in reserves will decide to retreat. It is a peculiarity of modern battles that all the mishaps and losses sustained in its course can be retrieved by fresh troops. The reason lies in the modern order of battle and the way in which troops are brought into action, permitting the use of reserves almost everywhere and in any situation. Therefore so long as a commander has more reserves than his enemy, he will not give up even though the battle shows signs of going badly. But once his reserves start to become weaker than the enemy's, the end is a foregone conclusion.

[…]

In all cases in which the above conditions have not already led to a decision, and in particular where the main objective is the destruction of the enemy's forces, the moment of decision comes when the victor ceases to be in a state of disarray and thus to some extent ineffective; in other words, when the successive application of force as discussed in Chapter 12 of Book III, is no longer advantageous. That is why we designate this point as central to the strategic unity of the engagement.

[…]

The smaller the proportion of troops in actual combat, and the larger the proportion that contributed to victory merely by being present as a reserve, the less is a new enemy force likely to deprive us of victory. The commander and the army who have come closest to conducting an engagement with the utmost economy of force and the maximum psychological effect of strong reserves are on the surest road to victory. In modern times the French must be credited with great mastery in this respect, particularly under the leadership of Bonaparte.

Archduke Charles was the brother of the last Holy Roman Emperor, and one of Austria's most successful generals during the long wars with France, the following is point 9. from his 1796 observations for subordinate commanders

More than usual thought must be given to the replacement of the meetings and divisions in the fire, because it is an essential observation that often decides the victory. Anyone who has lived in several campaigns and hostile incidents knows what condition a troop is in that has been involved in a lively, heated and persistent battle for several hours.

The condition of the weapon, of the physical strength of man, which only permit a certain degree of exertion, make it necessary in this case to arrange a reserve. A fresh division, spurred on by the thirst for honor and the wish to see the matter decided soon, appears with renewed vivacity and fresh courage; - their fire is doubled, their advance is decisive. The exhausted enemy, who for the most part has shot himself, is desperate for victory and mostly gives way to this new impulse. This is also the reason for the benefit of the second echelon and reserves in every occasion.

Ardant du Picq was a French officer of the 1800 century, who fought in the Crimean War and was killed by the Prussians in the 1870 war. His treatise Battle Studies has been very influential on historians attempting to recapture the experience of combat.

Self-esteem is unquestionably one of the most powerful motives which moves our men. They do not wish to pass for cowards in the eyes of their comrades. If they march forward they want to distinguish themselves. After every attack, formation [...] no longer exists. This is because of the inherent disorder of every forward march under fire. The bewildered men, even the officers, have no longer the eyes of their comrades or of their commander upon them, sustaining them. Self-esteem no longer impels them, they do not hold out; the least counter-offensive puts them to rout.

He, general or mere captain, who employs every one in the storming of a position can be sure of seeing it retaken by an organized counter-attack of four men and a corporal.