go for:

2 GK
2 DC/DR
2 DC/DL
1 DC/DMC
1 MR/DML
1 ML/DML
1 AML/AMC
1 AMR/AMC
4 MC
3 ST

That is 18 players. You are allowed 4 more. Base the other 4 players on the formations you tend to play. If you tend to play with 4 defenders, get another 2. If you tend to play with 2 ST, get another ST. If you use the butterfly, get more AML, AMR, DML and DMR.

Also, MC and ST tend to not have multiple positions.