Can I fly to Luang Prabang and back to Chiang Mai on the same day for a visa exempt stamp?

Not really visa related

There is no amount of time you have to be out of thailand so you could fly there and back no problem. Although it does beg the question why would you fly to a country where you have to BUY a visa to get in versus flying to say Malaysia where you get stamped in for free.

There is no official limit to the number of 30 day visa exempt stamps you can get when entering by air, BUT remember if the officer at passport control thinks you are living here on back-2-back visa exempt entries they can always ask for proof of 20K baht (or the equivalent) in cash, proof of onward travel (within 30-60 days) and proof of a hotel booking.

There is only one flight per day LP CM LP so you would need to stay overnight in LB

I seem to remember that a visa to enter Laos at Luang Prabang Airport was quite expensive.