If you think about it, it is really simple. Na stands for Sodium. C stands for Carbon and O stands for Oxygen. Whenever it is a capital letter, it indicates a new element. The number next to the letter indicates how many atoms are there. This means that there are 3 atoms for O. If there is no number, then it indicates that there is only 1 atom. 1 for Na, and 1 for C, making it a total of 5 atoms. But to answer your question, there are 3 oxygen atoms.