Question

An energy level has a fixed number of electrons. But when electrons jump from ground state to their excited state...the number of electrons in the excited state should increase and in the ground state should decrease.... So the formula of 2n^2 would not work! !!

Answer 1

yes what you said is correct 2n^2 wouldnot work in that condition but electrons stay in excited state for shorter periods only(unstable state), they reach back to their ground state after the supply of energy to the electron is taken off.
It is not fixed that a energy level can accomodate a fixed electrons it can accomodate more than it also but not stable state, it reaches to its 2n^2 configuration which is stable