This feature isn't one of my favorites from Custom Commander (it seems odd these troops popping up from nowhere, as compared to the more natural Diplomacy recruiter running around and gathering folks via the Constable), but it works like so:
Every 24 hours, every walled center gets checked for a recruitment plan. There is then a randomized chance to get various tiers of troops, with castles much more likely to get high level troops than towns. The number of each troops is also randomized, from a minimum of 2 or 3 and the highest tiers to a max of around 20 at the lowest tiers.
Regardless of the tier or the number, you always pay the same amount.