Which of the subatomic particles determine the mass of an atom?

The number of proton and neutrons.

Explanation:

The mass of an atom is concentrated in the tiny nucleus. Protons and neutrons are massive particles while the mass of electrons are negligible.

It is a common thing to relatively compare the mass of these particles.

If the mass of proton is 1 unit

The mass of neutron would be 1 unit

The mass of electrons would be 1/1840 unit.

We see that electrons have very small mass or negligible mass compared to the neuleons.

This is why when calculating the mass number, we sum the number of protons and neutrons alone.