Bitcoin mining difficulty is an expression of how many hashes a miner needs to produce a cryptographic string, these strings then earn the right to add the next block transactions to the chain.
This is not accurate. Bitcoin difficulty determines how difficult it is to mine a block. But there is no way to know how many hashes a single miner needs to generate to solve a block.
It's possible that a miner with the hash power of only 1 MH/s can find a block.
This readjustment depends on how many miners are operating in the network and their combined hash rate in the mining network. This ensures the steady discovery of blocks.
It doesn't matter how many miners are trying to solve the blocks. There is no way to know that at all.