in my experience normally when you either don't need them or can't afford them.
Maybe I'm missing something about the original question, but you can just open the military screen and click on the Mercenary tab to hire them. There are a number of different companies each with slightly different troop composition and price. The cheapest are 75g to hire and 6g/month to maintain, though those prices may vary for different rulers.
Precisely. When you've got enough troops and are on the verge of winning you'll get a popup asking you if you want to hire. Or when you've finished moving your crusader armies from England to Jerusalem you'll be asked if you want to hire 1000 longbowmen from Lincolnshire...![]()