The number of electrons in a neutral atom of an element is equal to the number of protons, which is the element's atomic number. For example, if you find oxygen on a periodic table you will see that it is element number 8. This means that it has eight protons, and therefore eight electrons.
As you probably know, atoms can gain or lose electrons forming charged particles called ions. In order to find the number of electrons in an ion you need to know how many electrons have been gained or lost. If an atom has a positive charge it's lost electrons and if it has a negative charge it's gained electrons. The oxygen ion, O2-, has eight electrons because the neutral atom O had six and it had to gain two electrons to have a charge of 2-. Similarly, the sodium ion, Na+, has 10 electrons because neutral sodium has 11 (the same as its atomic number) and it had to lose one electron to have a charge of 1+.
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