Write out the mask in binary and apply it.
00001010 . 00000000 . 00000000 . 00000000 = 10.0.0.0
Mask = 18-bits, so the maximum (aka wild card mask) is:
00000000 . 00000000 . 00111111 . 11111111 = 0.0.63.255
11111111 . 11111111 . 11000000 . 00000000 = 255.255.192.0
The left most 18 bits represent the network, while the 1's represent the hosts on the network. To find the broadcast address, just maintain whatever the first 18 bits are in the original 10.0.0.0 address, and replace the following 14 bits with 1's. In order to get the next subnet, simply increment the left hand side:
00001010 . 00000000 . 00111111 . 11111111 = 10.0.63.255/18
00001010 . 00000000 . 01111111 . 11111111 = 10.0.127.255/18
00001010 . 00000000 . 10111111 . 11111111 = 10.0.191.255/18
00001010 . 00000000 . 11111111 . 11111111 = 10.0.255.255/18
and so on...
Similarly, simply change all of the host bits (right most 14 bits in this case) with 0's while maintaining the first 18 bits' original values to find the network address.
So, the last subnet would have to be, while keeping the first octet in tact (RFC 1918 address):
00001010 . 11111111 . 11000000 . 00000000 = 10.255.192.0/18
There are no subnets that begin higher than that, assuming a /18 CIDR.