my query is that what is the maximum no of host's are possible for Private network in class wise. i.e i want to know No of Private host's in CLASS A,B,C? how to calculate these no?
For classful networks, the available addresses are given by the Wikipedia page on Classful Networks.
Class A = 16,777,214
Class B = 65534
Class C = 254
The Wikipedia page also discusses the calculation:
The number of addresses usable for addressing specific hosts in each network is always 2N - 2, where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host portion for network address and the all-bits-one host portion as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the number of hosts is 254.
As Zac67 points out in the comments, /31 subnets differ in the calculation as there is no network and broadcast address. With /32 a single host is defined. These subnets are not available in classful networks but I'm adding them here for completeness.
It's worth noting that nobody uses classful networks anymore.