For principal quantum number n = 4 the total number of orbitals having l = 3

$\mathrm{n}=4\to 1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2,3{\mathrm{p}}^6,3{\mathrm{d}}^{10},4{\mathrm{s}}^2,4{\mathrm{p}}^6,4{\mathrm{d}}^{10},4{\mathrm{f}}^{14}$
So I = (n-1) = 4-1 = 3 which is f subshell contain 7 orbital.