It is the number of protons in the nucleus of an atom, which is also the number of electrons in a neutral atom.
For any element:
If you squint, you can read the homemade element flashcards Stephen made for the first four elements of the Periodic Table.
(Click the image above to download.)
If you squint some more (and use your imagination), you can double-check our arithmetic and see if we did the first 8 elements correctly!