You've seen 21 cats?! Yes, there are 21 cats in that pet shop! What's amazing, the 21st cat is pink!!!!!!!!!!!
If there is more than one cat, use plural form.
Okay, this is your second example.
This is fine because you have two good friends! Besides, they are the best ones!I have two best friends.
Now I'm a bit confused! Do you mean to say "It is the 21st cat"? If yes, you are just pointing out one certain cat!In Arabic we say that it is 21 cat, I think this what makes it diffecult for me to decide.