To determine the number of electrons in an atom, you need to know its atomic number. The atomic number represents the number of protons in the nucleus of an atom, and in a neutral atom, it is also equal to the number of electrons.
The atomic number of an element can be found on the periodic table. For example, the atomic number of hydrogen is 1, meaning it has one proton and one electron. Oxygen has an atomic number of 8, so it has eight protons and eight electrons.
In summary, the number of electrons in an atom is equal to its atomic number.