Welcome to the Org Meowlin (is that from Pokemon?)

The size of the units can go up to about 240 for peasants on huge... great for making cities grow faster by recruiting peasants in one and then disband them in another (grew Sparta very quickly using 2 other cities to 'feed' it).

10000 people would require multiple armies.

The most you could field is 240 x 20 = 4800 in a single army.