what is process of finding minimum number of confirmation for a transaction in LISK?

 that depends on you. The confirmations requirement is up to you on how safe you want your tx to be... Usually the greater the number of confirmations, the higher is the immutability.
I would take exchanges as reference. Binance takes 3 confirmations, bitrex if I'm not wrong 303...
If I would have to decide a # of confirmation for my project, I would check if the network is okay (most nodes on the same blockheight) and then wait for the first 10 confirmations (~100 seconds)

Can confirm that a number of exchanges use 303 as the number

Currently Lisk doesn't have finality, so there is no solid number how long exactly one should wait
so minimum would differ from nature of ur application. (if it's critical it should wait like exchanges, but if not 10 would be ok)