An atom does not need any electrons if it has outermost shell a full 1 electron in its more than 21 electrons in its more than 2 electrons in its

Atoms have electrons orbiting in their outermost shells.
Electrons reside in orbitals in their outermost shells. Reactivity of atoms depend on the number of electrons in the shells. Elements with few number of electrons in the shells react by donating the electrons and elements with large number of electrons in the shells react by taking in electrons to complete the shell.
Atoms with complete shells are stable / inert and they do not have the tendency to react.
answer is atom does not need any electrons if it has a full outermost shell.

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.