A nucleon is a particle in the nucleus, so a proton or a neutron.
Deuterium is an isotope of hydrogen. Hydrogen, being element number 1, has one proton in its nucleus by definition. Deuterium is specifically the isotope of hydrogen with one neutron in its nucleus.
So what we have is one proton and one neutron, for a total of 2 nucleons. This is why you will often see deuterium written as 2H
We can extend this to look at any other isotope of any other element Take 7Li for example. Lithium is element 3, so 3 protons are in the nucleus. The 7 means there are 7 nucleons in the nucleus, so 4 must be neutrons.
Here is a sample question to check if you understand whats going on:
What isotope of bromine has 46 neutrons?
