21's are traditionally done by performing the bottom half of a bicep curl (7 reps), then the top half (7 reps) and then the full reps (7).
This is a one way to switch it up to give the muscles a new stimulus:
You start by performing the full 7 reps first.
Then complete 7 half reps from the bottom.
Then finish off with 7 half reps to the top.
And whilst these are called "21's".... because of the 7,7,7 reps - there is no reason why they cannot be done with other rep schemes. For example, if you want to do 5, 5 and 5. Or 10, 10 and 10... there's nothing stopping you.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!