What is one place?
Anyway, before railway and trucks there were severe limitations on how many men and horses you could feed on a given area (Martin van Creefeld has written a good book on logistics over the times) - in Napoleonic times the individual Armycorps (10-20.000 men) so had to march separately but should unite briefly in case of battle. With depots laid out previously or very rich country you of course could count on staying together for longer but that would also severely limit your operational mobility. On a pre industrial battlefield each side rarely exceeded 150.000 men and some tens of thousands horses and in larger battles like Leipzig the whole thing was about uniting the individual armies of 70-150.000 men just in time for battle, not too early and not too late. Before Napoleonic times field armies rarely exceeded more than 50.000 men and often much smaller.
When Napoleon sent 500.000 men into Russia in 1812 they were spread over a very large area but really was beyond what the contemporary logistic and operational system could handle.
In some ancient sources you can some times see refrences of much bigger armies, but I would be VERY careful to believe them.
The German force gathered for Barbarossa was more than 3 million men, and that is probbaly the record for one coherrent operation, but they spread over 1000+ km front so you really couldn't call it "one place".
The Red Army in late WWII probably had more than 3 million men in Europe but they were spread over many individual operations. The attack on Berlin in 1945 however involved some 2,5 million Soviet soldiers, that might a good bid on biggest army in one place.
If we have to be strict however "Largest army in history in one place" I guess we would have to measure in denisty: how many soldiers pr square meter? In that case it could be an Austrian Battaljon Mass during the Napoleonic wars. Here companies moving in column (one, two or three company front) pressed the ranks very closely together and the outwardly placed men pointing their bajonets out of the formation. It was very effective vs. cavalry and was quick to form but also quite vulnerable to artillery fire. It would however be a good bid for most soldiers pr. square meter/feet.
