Did they go 1, 2, 3... And so on, multiply e.g 5 tents/men in 7 rows or "that right there is about 100 plus that there which seems to be double" and so on.
Hi, could you specify a region and time period? Are you referring to ... European armies during WWII? Ancient Greece? Thanks!
Perhaps not count literally the entire army, though armies did and do still deploy scouts and pickets to guesstimate the size of an enemy force. There's no way of completely 100% accurately observing the enemy's full strength, but if you (for instance) know that a regiment of 300 men has a certain type of flag they carry as a standard, you can just count the number of those flags you see and extrapolate from there. If you see three to five regiments you could say you see a brigade, if you see seven to ten regiments you could say a division, and so on and so forth.