A pro-football team needs

at least 4 impact players besides the quarterback & a left tackle. I'd build a defense first, followed by an offensive line. Say, on defense, an impact lineman, linebacker, and defensiveback. The either a running back or wide receiver. So, that gives a team six key players. Add another six (@b-level) + six (@B- level).
Ideally, when you substitute a defensiveback in a nickle-situation we also change in the line/lineback area where the original substitution came from. So, 11 - 1+1 = 9 + 2 = 11. total 13.
offensive line needs 8-9.