Pre iOS 10, I played a game that relied heavily on game center for in game friends and i had built up around 40 friends. When 10 did away with the game center app, you couldn’t add anymore but the existing friends were still there. I no longer play that game and i had game center turned off until i started my small town. By saving it to a different game center, it showed up in my friends list in township even though my list was full.
Three other of my game center friends popped up on my list but they seem to be inactive. Maybe they no longer play, maybe they now play on android or maybe they just have game center turned off.
So, my you still have game center turned on? If not, by turning it on, do new (old) friends show up?