How many protons are in an element with an atomic number of 8 and a mass number of 18

8 protons because a element always has the same number of protons as its atomic number

There are 8 protons in an element with an atomic number of 8. Atoms are numbered after their number of protons. The number of protons is what defines the element. If the number of protons were to change, the element would not be the same, but another different one. That's why the number of protons is single to each element and they are numbered after them.
The atomic number is many times represented by the letter Z.

Not sure what you mean by this question