The transmutation equation for this process is:
(α) ²He^4 + (O) 8^16 -> (N) 7^15 + (H) 1^1.
In this reaction, an alpha particle and an oxygen-16 atom combine to form a nitrogen-15 atom and a proton, while releasing energy.
For the standard isotope Oxygen16 O = 16 but atomic number 8 therefore 16-8 = 8 - so 8 neutrons
To find the number of neutrons, subtract the atomic number from the atomic mass of the element. The atomic mass of an element accounts for both protons and neutrons in the nucleus. The atomic number represents the number of protons, so the difference will give you the number of neutrons.