Would it be possible to use quantum computers to solve for the unified theory of quantum gravity?  How many qubits would that require?

I don't see why or how.

If it is possible for a computer to solve it but I don't see why a quantum computer would do it any better/differently from another computer of equal power.

All computers, no matter the technology used, have a common property

Garbage in ... garbage out.

IOW, if you don't know how to solve the problem, you cannot program a computer to do it.

16 minutes ago, MigL said:

IOW, if you don't know how to solve the problem, you cannot program a computer to do it.

Not entirely true anymore. What previously would’ve taken 8 years for a PhD student can now be finished in half a second. 

its not as simple a matter as mere calculations in order to renormalize gravity you must eliminate any divergence. a common method used is via a regulator operator. However with gravity we do not know any upper bound ( ultraviolet boundary). Good example being the singularity of a BH.