This is a problem for modern battles as well. It always looks like you’re fighting a million and one enemy. Also, the bragging factor almost obligates historians to multiply the feats of their own heros and leaders by multiplying the number of foes they vanquished.
As for feeding all those guys, for ages it’s been a matter of everyone fending for themselves or their own personal troops. The crusaders ravaged the land they passed through just by eating everything in sight. Napoleons Republican army supplimented their provisions with victuals acquired from the surrounding countryside.
One of the things an ancient army commander could do to ease food problems was to spread his army out over a wide area as it advanced. This reduced the ravaging that the land had to endure. Important if it was your own countryside that you were marching over. Many armies would split their forces in order that each of the sub-units could take a different route to the destination. There would be more food for each soldier, the routes would be less crowded, and the spread of disease could be reduced.
Another measure would be to create advanced bases of provisions along the route of march. This kept the men from having to transport their food along the way, but it was risky to have all that stuff sitting around if there was any chance of the enemy getting it or destroying it. A feature that marked a well-organized army was a group of officers whose jobs was to ride ahead of the army to organize provisions for the following forces. Once again, this was difficult to do when advancing across enemy territory. The Romans could do this for legions leaving Rome for the frontier because the roads allowed the army to cross their own territory quickly without trampling everything in sight and they had a network of towns where provisions could be acquired and secured before the arrival of a legion.
A good commander could play the food card against his opponent. The barbarous franks northwest of the post-Roman Gothic territories were formidable fighters and could amass a huge army. But a smart gothic general could cut their forces in half just by delaying battle. The Franks would get hungry (and bored) and melt away home to find food.