I think that how many Crusade-only unit you can get will depend on the zeal level of the province where you start the Crusade. Higher zeal = more and better troops. It may depend on other things such as the King' stat (higher influence and peity), level of developement of the province. I have had 5 Crusades so far, but only get one knight (Knight of Santiago). The others get either 2 order foot soldier or 1 order foot soldier and 1 fanatic.

Still, I am not so sure what will affect the zeal level. I have one province: the King with very good stats + around 900 garrison troops, 3 assasins, 2 spies, 2 bishop, 1 cardinal, 100% Catholic, best level of developemet (has church, monastary, reliquary, catheral), zeal = 37% (stay there for nearly 20 yrs now). Compare to a newly conquer province: OK stat governor + 240 garrison troops, 2 assasins, 1 spy, 1 bishop, 2 cardinals, 100% Catholic, newly developed (has church & monastary), zeal = 97% (go up from around 20 and stay at 97 for a few yrs now). So, what would be the key affect to the zeal level?