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The azimuthal number l must be smaller than n, and it fixes the shape: 0 is s, 1 is p, 2 is d, 3 is f. Each subshell has 2l+1 orbitals, one for every magnetic quantum number from -l to +l, and the Pauli principle allows two electrons per orbital. So 3d has five orbitals with ml from -2 to +2, holding ten electrons.
Orbital count
orbitals = 2l + 1
Electron capacity
electrons = 2(2l + 1)
The radial wavefunction needs at least l + 1 nodes-worth of room, so a 2d orbital cannot exist. Values above n-1 are clamped here.
It is always +1/2 or -1/2, which is what puts the factor of two in the electron capacity.