Let’s say I played Ajani, Caller of the Pride and then played Ajani, Wise Counselor. Would I be able to use Caller of the Pride's -8 ability to give me, say, 20 creatures and then use Wise Counselor's +2 ability in order to gain life for each creature on the field? Would I be able to do that in the same turn or would I have to use one wait a turn then play the other?
306.5d Each planeswalker has a number of loyalty abilities, which are activated abilities with loyalty symbols in their costs. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”
The rules only say something about not activating a single planeswalker's loyalty abilities twice in a turn, not about a hard limit of one loyalty ability activation per player. Therefore, you can use one Ajani's ability first and then the other's ability in the same turn.