Each orbit surrounding an atom is allowed A LIMITED NUMBER OF ELECTRONS.
The number of orbit that an atom has is determined by its atomic number, the higher the atomic number the higher the number of orbit in the atom and each orbit has different energy level. Each orbit can only take fixed number of electron. The first shell can only take two electrons while the subsequent shells can only take eight electrons. When an orbit has taken the highest number of electron possible, the remaining electrons are moved to the next shell.