I'm not sure what the limit on one particular unit type is, but I imagine it's at least 100 (probably 255) and you can have quite a few different types of units in your army (more than 10 I believe), so that's at least 1000 troops right there. You could easily fill up a battlefield without having to have "infinite" army size. Also, the problem with different types of units comes in when taking them prisoner, where there can't be more than 5 types (due to the fact that only 5 areas of memory are allocated). There's a similar limit on the different unit types in your army and also different unit types you can "rescue" after a battle, but I'm not sure what it is off hand (10 or 15 likely).