First - the number of merc points the Order members get is divided amongst them. So, if you add an order member - the rest lose a few merc points.
2nd - The number of troops available to each order member does not change - I think. I'm pretty sure that each order member gets to buy the same number of that merc type. Example - knights templar. I build a hut, the order gets 100 more templars available to buy. 100 for each order member - no matter how many order members.