The isotope sodium-20 has how many protons?
The most important thing to remember here is that all the isotopes of a given element must have the same number of protons.
For a given element, the only difference between its isotopes is given by the number of neutrons that are located inside the nucleus. The number of protons is always the same for all isotopes that belong to a given element.
In your case, a sodium-20 isotope must have
An atom that has
Now, the name of the isotope contains its mass number, which si the number of protons and neutrons located inside the nucleus.
#color(blue)(ul(color(black)("mass number" = "no. of protons" + "no. of neutrons")))#
In this case, you can say that a sodium-20 isotope contains
#"20 protons "#and #" "20 - 11 = "9 neutrons"#
inside its nucleus.