If I could get to the point where each defender on my roster could play all 3 defensive roles, that's how I'd do it. Then have 2-3 DMCs (one who can play MC and AMC; the other two can play DML, DMR). 3-4 MCs (again, 2 who can play MR/ML, and two who can play DMC/AMC). Then two attacing mids (who can play AML/AMC/AMR) and one who can play ST. Two strikers and two keepers puts you at 20 players (2 open slots on the squad).

Basically, I like my players to be able to move laterally across the field (mostly) versus moving vertically up/down the field.