Mutually Assured Destruction depends on both sides having enough nukes to completely destroy each other. If both, or even just one, side(s) only have a handful of nukes then mutually assured destruction losses the "assured" part. At the point nukes actually become a viable battlefield weapon. To maintain the proper balance of terror both sides need at least a few hundred weapons each, though 1,000 - 2,000 would be preferable. The biggest problem with MAD is making sure it doesn't just become a fluff measuring contest like it did during the Cold War. The solution to that is treaties that limit the number of devices each side can possess while still giving everyone enough devices to ensure the "mutual" part of Mutually Assured Destruction.