It is sort of 2 questions rolled into 1 - 

I have to calculate the theoretical yield of meso-hydrobenzoin when
50mg of benzil is rduced with NaBH4 and H2O. So according to what I
thought was a balanced equation, it appears that 2 moles of benzil
form 2 moles of benzoin. Since the NaBH4 is in excess, the 50mg should
be limiting reagent. Given that, I found that moles of benzil in 50mg
and that wold equal the moles of hydrobenzoin, which would give me
51mg. But the text says we should 35 mg - what am I doing wrong?

I have the reaction as: 4(R2C=O) + NaBH4 gives (R2CHO)4B-Na+
             next step: (R2CHO)4B-Na+ + 4H2O gives 4 (R2CHOH) + Na+B(OH)4-

I have not been able to find any balanced equations for this reaction
online so I don't know if my stochiometry is correct now!

Thank you very much!

This reaction is more complex than you are taking into account.  The
question asks for the yield of *meso* hydrobenzoin.

You might also want to check the structurs/formula of "benzil" and
"hydrobenzoin."  I can't tell for sure from the way you've written the
reaction, but I think you are missing some bits of the molecules.