As always, that depends on the purpose of that particular element of the reserves.
If that element is only part of a training organization whose purpose is to train tankers and can sequence the training of 5 crews with just 2 tanks then fine.
If that element is part of an armoured regiment whose purpose is to provide a fully trained and deployable armoured regiment to expand a deployed force with a full regiment, or to sustain one depleted by personnel and tank casualties, then all five tanks are essential.
Context matters.