Answer
64 fundamental fermions
Work Step by Step
The fundamental fermions include quarks and electrons.
In $H_2O$ there are 2 hydrogen atoms consisting of 1 electron and 1 proton each. There is also 1 oxygen atom, which has 8 electrons, 8 protons, and 8 neutrons.
In total, there are 18 nucleons (3 quarks each) and 10 electrons. The total number of fundamental fermions is 3(18)+10=64.
