Damn, if I'd known I could just get Dan's football wisdom on demand, I'd have asked this question years ago!
21 personnel - first number is backs in the backfield, second number is TEs.
So 10 is 1 back, no TE.
You work out the no. of WR by exception. So in 21, it's two WR, in 10 it's 4 WR.
Dan, hope you don't mind I took the easy question for you 😎