The rule of thumb usually is to have 2 defenders for each ST the opponent uses, and 1 defender or defensive mid for each AM they use. so against a 4-5-1 v style, with 1 ST and 2 wingers, the 1 ST is marked by 2 DC and each winger has a DL/R. as for DML/R, most people use them to attack the flanks, so they have red arrows on them, meaning they are more midfield based then defense