H-bridge made of MOSFETs
This assumes an on-off type H-bridge and not a linear H-bridge used in  bridge tied audio amplifiers.

How much?

If you ignore protection features (required to limit gate-source voltages when the supply exceeds the maximum gate-source voltage), to efficiently drive current through the load requires these gate voltages to be present: -

And clearly, the difference in voltage between the two driven gates for the MOSFETs that are activated is just the supply voltage.