The Basics

Electrons are held within orbitals, which are held within sub-shells, which are held within shells.
Orbitals can only hold up to two electrons.
Sub-shells can hold varying numbers of orbitals, & therefore varying numbers of electrons:
's' subshells hold up to one orbital (two electrons).
'p' subshells hold up to three orbitals, often labelled x, y & z. (six electrons).
'd' subshells hold up to five orbitals (ten electrons).
A shell / energy level can hold varying numbers of sub-shells:
Shell 1 holds only one 's' subshell.
Shell 2 holds one 's' sub-shell and one 'p' sub-shell.
Shell 3 holds one 's' sub-shell, one 'p' sub-shell and one 'd' sub-shell.
Shell 4 and onwards follow this same pattern as Shell 3.