Look at the rank of general and the establishment of the army. Ancient armies often had their own fixed establishment. For example, in the Qin and Han dynasties, the establishment of five generals and centurions was often adopted. In the Ming dynasty, every 500 people served as guards. Through the establishment of generals and troops, it is easy to estimate the approximate number of troops sent by the other side. The military establishment of each dynasty is different. You can also look at the positions of local generals, because different positions lead different people. Through rough calculation, our generals can get how many military forces the other side has received.
Look at the station. In ancient times, fighting had a set of fixed procedures. The layout of the army, the size of the camp and the flag of the general are all factors that can be used to judge the strength.
Look at logistics. In ancient times, many generals would judge how many people there were according to the number of pots and pans of each other. Judge the strength by counting stoves, and integrate other ways of grain and grass transportation and drinking water. Many times, we need to catch a few prisoners of the other side and ask them to come back for more accurate analysis.
The above methods can only be roughly estimated. If you want to know the real number, you have to ask the Lord!
I hope the above answers can help everyone. If you are satisfied, please accept them. Thank you.