It's pretty simple...
First number denotes backs (RB, FB)
Second number denotes tight ends
The rest are receivers. The 5 lineman and the QB.
21 personnel is 2 backs, 1 TE, 5 lineman and QB = 9 players, which means there is 2 WRs.
01 personnel (1 personnel) is 0 backs, 1 TE = 4 receivers.
You...