Answer: Number of shells: 5.
Step-by-step explanation:
Any orbital can hold a maximum of 2 electrons with opposite spin. The first shell has one 1s orbital and contains 2 electrons. The second shell contains 8 electrons: 2 in a 2s orbital and 6 in three 2p orbitals. The third shell has 18 electrons: 2 in a 3s orbital, 6 in three 3p orbitals, and 10 in five 3d orbitals.
The 5th shell has 5s, 5p, 5d, 5f, and 5g orbitals. An s orbital can hold 2, p holds 6, d holds 10, f holds 14, and g holds 18 (which you are not expected to know for this class). If you add them up, you get 50 electrons that the 5th shell can hold. This also follows the 2n^2 rule.