I am Zephyrianr on Game Center too, add me if you want.
By the way it IS possible to add multiple people at once, but I think you need to enter in the names separately i.e. you can still send out multiple requests at a time.
Q: Do any of you, guys, have more than 500 friends?
I have exactly 500, and the number stopped increasing no matter how many more friends I approve. Yes, I still get several requests a day, and I approve all of them. But the number is still stuck on 500.