As of now, the perks of adding friends are only for those who fight/ do Wizarding challenges together, where as they state “Friends provide social benefits, such as bonus Wizarding XP and Wizarding Challenge XP when you engage in Wizarding Challenges together”.
Thus, for now, you don’t need that many friends, but I believe like Pokemon Go, they will add more benefits in future.
However, you should still add friends, because you can complete achievements and earn XP, as well as free coins.
Eg. Have 1 Friend in your Friends List – XP +300
I will update this post when there are more perks revealed or as I discover them.
If you want to know how to add friends or find friends codes, or even add your own codes to my list, check out my previous post: Wizards Unite Add New Friends
To be updated…