Electrons in the same orbital have the same first three quantum numbers, e.g., $$n=1$$, $$l=0$$, $$m_l=0$$ for the 1s subshell. Only two electrons can have these numbers, so that their spin moments must be either $$m_s = -1/2$$ or $$m_s = +1/2$$. If the 1s orbital contains only one electron, we have one $$m_s$$ value and the electron configuration is written as 1s1 (corresponding to hydrogen). If it is fully occupied, we have two $$m_s$$ values, and the electron configuration is 1s2 (corresponding to helium). Visually these two cases can be represented as