Millions! A rough estimate might be 25 million. The interviewer really wants to hear how you come up with your estimate. Reasonable logic might be that manholes serve the sewer system which needs to serve homes and businesses. If the U.S. population is roughly 320 million and there are roughly 2.5 people per household, there are about 128 million households. One manhole on the sewer system might support 5 homes which would mean that there are about 25 million manhole covers.
Feel free to pull out your calculator during the interview or ask for a calculator!
"Likely over $20 million. A great way to verify this would be though the EPA or by understanding how many households per manholes, then work the math."