How many electrons in an atom could have these sets of quantum numbers? n=2?

A maximum of 8 electrons can share the quantum number  n = 2.

Principal Quantum number has a symbol of "n". It tells you the energy level on which an electron resides. You need to determine exactly how many orbitals you have in this energy level before you can determine the number of electrons that can share the value of n.

The number of orbitals you get per energy level can be found using this formula:

no. of orbitals=n²

Each orbital can hold a maximum of two electrons, the formula would be:

no. of electrons=2n²

Using the given formulas:

no. of orbitals = n² = 2² = 4

no. of electrons =2 * 4 = 8 

They are all decomposition reactions.