Answer:
To find the number of neutrons in an atom simply calculate the difference between the atomic mass of the atom and the number of protons in the atom. You can find the number of protons in an atom by looking at it's atomic number. The reason this works is because atomic mass is the amount of protons and neutrons. Subtracting the protons from that amount leaves us with the neutrons.
Example: Let's look at Arsenic (my favorite element lol) which has an atomic number of 33 and atomic mass of about 75 (always round to nearest whole number) which means that the number of neutrons is 75 - 33 = 34.