How to calculate the subnet number

number of total subnet created=2^x;x=like in class c we have 24 network bits in class c
a/c to class c it should be 192.168.5.139/24
                    255.255.255.00000000
but insted of this we have
    128 64 32 16 8 4 2 1
     1           1   1

that is we have 192.168.5.139/27
                11111111.11111111.1111111.10001010
with 3 extra bits

anyway now you have to do conversion (in you head). Memorize this
128 = 1
192 = 2
224 = 3
240 = 4
248 = 5
252 = 6
254 = 7
255 = 8
 this will lead to 255.255.255.224
2^3rd power = 8 bits available for network portion.
8-3=5 bits are left for host portion.
As for calculating your network numbers
I had 5 of those here.  2^5 = 32.  So every 32'nd number is my new network.
0, 32, 64, 96, 128, 160, 192, and 224.