How many neutrons does helium-7 have?

Helium has 2 protons.

Number of neutrons = atomic mass - number of protons

= 7-2

=5

Notice the pattern in these various isotopes of helium. The number of protons does not change, but the neutrons do. The total of the protons and the neutrons = the isotope of that helium.

There are 2 stable isotopes of helium:

Helium 3 has 2 protons and 1 neutron.

Helium 4 has 2 protons and 2 neutrons.

The next isotopes are radioactive:

Helium 5 - 2 protons, 3 neutrons.

Helium 6 - 2 protons, 4 neutrons.

Helium 7 - 2 protons, 5 neutrons.

Helium 8 - 2 protons, 6 neutrons.

Helium 9 - 2 protons, 7 neutrons.

Helium 10 - 2 protons, 8 neutrons.

theoretically, since helium has two protons and two electrons, it would need 5 neutrons to be helium-7

5 neutrons.

this could of been easily answered by doing a few minutes of looking in wikipedia. Instead you find it more convenient to just have someone answer your question rather than discovering the answer for yourself. Let me suggest that if you can't figure this question out and you are too lazy to do any work, you probably don't have much chance of passing chemistry.