Yes, that's correct It's just a shorthand counting the number of bits set most significant to least significant....each value in the 'number netmask' is 8 bits....so 255.255.255.0 = 8+8+8+0 = /24
And to make it plainly obvious, 255 = 11111111 in binary. where the 8 is from is the 1 bits in the subnet mask, so a /24 is: 11111111.11111111.11111111.00000000 or 255.255.255.0
since there are 24 ones, the mask is /24. Alternatively, you can consider the host bits, which are to the right of the last one bit. The host portion gives you the number of IPs in each subnet. In the case of /24 that is 256 (less the bcast and mcast address) or .0-.255.