Casualty numbers were actually fairly low, much lower than we might expect from watching Hollywood movies. One historian estimated that, in Roman civil wars, the victorious side might generally suffer a roughly five-percent casualty rate, while the losing side would suffer a bit higher, around sixteen-percent. These numbers are estimations, as it is hard to know with certainty. Most battles against foreign enemies were probably even lower, as in most cases the Roman army greatly overwhelmed the enemy. Of course, there are excpetions, such as the Battle of Cannae, where three-quarters of the Roman army was reportedly captured or killed by Hannibal's forces, or the Battle of Adrianople, where one-third of the Roman army, and the emperor Valens, was destroyed by the Goths. Furthermore, during the Battle of Teutoberg forest basically the entirety of Varus' army was annihilated. These battles, although not unfrequent over Rome's long history, are outliers from the generally low numbers of dead.

As for your third question, I do not believe many recruits would have been picked up while the army was actively campagining. Because of the low casualty rates, it would not have been necessary to be replenishing numbers while an army was out marching during the few summer months when campaining actually happened. During the winter months when the armies were garrisoned, soldiers would have been retired and recruited as normal to keep the army's numbers up. If massive losses were taken, such as during the Battle of the Teutoberg forest, the campaign simply ended for that year with no army left to continue it. After Cannae and Adrianople, new armies were created through a combination of recruiting new soldiers and transferring soldiers from other Roman territories.