The ampacity would be 20 amps, and you could legally cram them into a 3/4" EMT. I don't know of anyone who would try this, but it could be done. If you were to pull an equipment grounding conductor as opposed to using the raceway as the EGC, a 1" EMT would be required.
The ampacity of ten #10 THHN copper is 20 amps, as Ryan has said. And yes, you can put eleven #8 THHN into a 1 1/2 inch conduit. But in that case, the ampacity would be 27.5 amps (i.e., 55 amps times a 50% derating factor). However, did you mean to say ?10 circuits?? A circuit has 2 wires, so 10 circuits would have 20 wires. Please note that you could still put twenty #8 THHN and one #8 EGC into a 1 1/2 inch conduit, and the ampacity would still be 27.5 amps. But as Ryan said earlier, that would be a difficult installation.
I can?t comment on whether the engineer is ?OK,? because you did not tell us enough about the proposed design (e.g., what is the required ampacity, and how long is the run?).
Dereck and Bob are right, but I?ll put it in different terms. The NEC provides the minimum requirements for a safe installation. If she wants to go above and beyond the minimums, then that would not be in violation of the code. However, presuming the circuit requires no more than a 20 amp?s worth of conductors (you still have not confirmed this), her assertion that you MUST use # 8 wire and 1 1 /2" conduit is not correct. A more accurate statement is that you must use at least #10?s, and you may, if you wish, use a larger wire.