The total number of subshells is equal to the shell index number.
The "n" is a quantum number (index number) tells us not only which level, but how many subshells are possible.
If "n" = 1, there is only one possible subshell (1s)
If "n" = 2, there are two possible subshells (2s, 2p)
Any "n" can only contain that many possible subshells