Power is more complicated. By design, Bitcoin has one block created by one miner every 10 minutes on average. All other work by all other miners is discarded and unused. So no, only a single miner is required to sustain the network. But, there is a economic incentive to add miners to the network when the price goes up, because even though only one miner gets rewarded per block, that reward is worth quite a bit. Currently approximately $1M of bitcoin are mined per hour.
There is no specific value at which all mining is unprofitable because mining difficulty is dynamically adjusted to maintain a 10 minute block generation rate regardless of the number of miners.
Power is more complicated. By design, Bitcoin has one block created by one miner every 10 minutes on average. All other work by all other miners is discarded and unused. So no, only a single miner is required to sustain the network. But, there is a economic incentive to add miners to the network when the price goes up, because even though only one miner gets rewarded per block, that reward is worth quite a bit. Currently approximately $1M of bitcoin are mined per hour.
There is no specific value at which all mining is unprofitable because mining difficulty is dynamically adjusted to maintain a 10 minute block generation rate regardless of the number of miners.