YOu do max out at 25 animals. HOwever if you have less than 25 and it still stays it's full, leave the animal be, on iOS force quit the game by double tapping the home button then flicking the preview off the screen. SIngle tap the home button to exit the mode and reopen the game.

Please Help.
Thanks !

I didn't find the right solution from the internet.
References:
-http://forums.bongfish.com/forum/main-category/smurfs-village/news-and-updates/191560-how-can-add-a-new-animal-to-my-sanctuary-if-i-keep-getting-the-message-that-it-is-full
- Performance Management Video Examples