How many hydrogens do you have to fuse together to make one helium?
None. Hydrogen and Helium are base elements.Helium is helium.Chemically yes, but with Nuclear Physics in stars:H + H --> D + e+ + v + 0.42 MeV, H + H --> D + e+ + v + 0.42 MeV, D + D --> He3 + n + 3.27 MeV (50%)H + H --> D + e+ + v + 0.42 MeV, H + H --> D + e+ + v + 0.42 MeV, D + D --> He3 + H + 4.03 MeV (50%)H + H --> D + e+ + v + 0.42 MeV, H + D --> He3 + gamma photon + 5.49 MeV, H + H --> D + e+ + v + 0.42 MeV, H + D --> He3 + gamma photon + 5.49 MeV, He3 + He3 --> He + H + H + 12.86 MeV (100%)H + D --> He3 + gamma photon + 5.49 MeV (100%)D + D --> He3 + n + 3.27 MeV (50%)D + D --> He3 + H + 4.03 MeV (50%)D + T --> He + n + 17.59 MeV (100%)T + T --> He + n + n + 11.33 MeV (100%)Four or two, depending on the kind of hydrogen isotope you are burning. As you can see in the equations above burning four ordinary atoms of hydrogen to ordinary helium is a complicated and slow process compared to burning two atoms of various isotopes of hydrogen, however a star has such tiny quantities of deuterium and tritium that depending on them for fusion helps little.