There is no max. of friends, but you can only use 1 of each pack a day (you will find sometimes when more than one is sent you will have it available the next day)

Unless you have tons of greens, for PT, I would not make an SA on anyone unless they are a super trainer, you won't be able to keep up his stars. Trust me on this one! The SAs are nice and all, but it is a HUGE hit in Q unless you are willing to put ~100 greens on the player to keep him up.
If you are willing, look at his current attributes and pick the SA that best matches his higher skills.