Can we calculate the number of miners,that we will need in private blockchain in advance and how? for POW algorithm
1 miner is all it takes on a private blockchain, as the difficulty of mining a block is adjusted so that the average sits between 10 - 19 seconds, as described in "How does the Ethereum Homestead difficulty adjustment algorithm work?".
If you are worried blocks will be mined too slowly, you can change the initial mining difficulty on your private chain as described here:
You can set the genesis block difficulty in your private network using the init parameter, but the difficulty will eventually be adjusted according to the algorithm described above.