please explain how to figure out subshells

Question

The number of subshells is the same as N.
If N = 1 we have 1 subshell
If N = 2, we have 2 subshells, the s and p
If N = 3, we have 3 subshells, the s, p, and d.
If N = 4, we have 4 subshells, the s, p, d, and f.

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