What is the minimal number of miners that can keep a coin alive?

I'm curious about this (for any coin). In general, the trend for alt coins is the price keeps dropping. At some point, most people will stop mining a particular coin.  If there is only one miner left, then no one will be verifying his finds. Can two miners verify each other and keep the coin going?  I think different coins have different requirements on how many verification is needed. But can the one miner repeatedly verify the same transaction and count as multiple verification?