I tend to have:
GK: one main team, one oldie and one future prospect
LB: one main team, one versatile and maybe a back up
RB: one main team, one versatile and maybe a back up
CB: two main team, one versatile, one back up, one future prospect
CM: three interchangable main team, one future prospect and one versatile
AML/ML: one main team, one future prospect, maybe a back up
AMR/MR: one main team, one future prospect, maybe a back up
ST: three interchangable main team, one back up, one future prospect.
With the versatile players it could be like Santon for example who can play anywhere up the right, so he would cover the versatile spot for all those positions, or maybe a Cm who can play AMC and DM or a defender who can play RB and LB equally efficient.