Question

How many neutrons are in an atom

Answer 1

That number is different in the atoms of almost every element,
and it's also different in the atoms of different isotopes of the
same element.

-- An atom of Hydrogen has no neutrons.

-- An atom of Uranium-235 has  142 neutrons in its nucleus.

-- An atom of Uranium-238 has  145 neutrons in its nucleus.

Answer 2

It depends on the number of protons and the atomic mass. In order to find the number of neutrons, subtract the number of protons from the atomic mass and there's your answer.