Count the number of 1 bits after the last decimal point. Call this number n.
There will be 2^n subnets and 2^(8-n) - 2 hosts per subnet. (You can mutilply the number of subnets by the number of hosts per subnet to find the total number of hosts.)
/25: 255.255.255.128, 2 subnets, 126 hosts per subnet
/26: 255.255.255.192, 4 subnets, 62 hosts per subnet
/27: 255.255.255.224, 8 subnets, 30 hosts per subnet
/28: 255.255.255.240, 16 subnets, 14 hosts per subnet
/29: 255.255.255.248, 32 subnets, 6 hosts per subnet