comparison/validation of greiner tubes

We are switching to Greiner tubes from BD. We average about 150 RBC transfusions per month. We use Ortho gel for ABSC and ABID and tube testing for everything else with Immucor reagents. What would be considered an adequate number of specimens for a parallel study? Would every test we perform have to be done? Can the number of specimens vary with the type of test? When we rarely use tube testing for ABSC or ABID and when we do comparisons every six months, we have to change the 0.8% cells to 3% which is time consuming. We don't have the staff for exhaustive studies.