Whaddya mean?
You need a full crew for any tank, size (and roles) of crew are determined by tank, you can only switch crew between tanks of the same nation and to the same role (a loader cannot become a driver or a radioman and so on). Moving crew to a different tank requires that they be retrained to it, loss in skill % depends on the type of switch - retraining to a different tank of same class means low penalty, to a different class - high penalty.
