Oxygen has #16 so it has 16 electrons.
By subtracting is the same as the atomic number.
To determine the number of electrons in an element, you can look at its atomic number. The atomic number represents the number of protons in the nucleus of an atom, which is also equal to the number of electrons in a neutral atom. However, it seems like there is some confusion in your statement.
The atomic number of oxygen is actually 8, not 16. Oxygen has 8 protons in its nucleus, and in a neutral atom, it also has 8 electrons orbiting the nucleus. The atomic number is specific to each element and is found on the periodic table.
So, in this case, by subtracting 8 (the atomic number of oxygen), we get the number of electrons in an oxygen atom, which is also 8. However, it's important to note that this subtraction works in this specific case because the element is oxygen. It is not a general rule that applies to all elements.