So you are verifying that one unit will pass a test after multiple uses? Is this for a wear test?
Whatever unit(s) you use, I think that they will define acceptable tolerances, won't they? With one unit, then you have verified that units made to those exact specifications (no tolerances) will pass the test? If you use a few units then you have verified that units made within the range of tolerances represented by those few units will pass the test?
PS I'm not adding to this discussion, but pursuing Design Engineering for Dummies, so nobody needs to knock themselves out to enlighten me. I won't ever even do any of this stuff myself. Just have an inquiring mind that wants to know.