Answer: 5 sheets
I previously thought it was 20 sheets, but I was misguided thinking you can mail 3.5 ounces with one stamp. But it’s ONE ounce.
Here’s how I figured it out:
500 sheets of paper = 5.3 pounds
5.3 pounds = 84.8 ounces
84.8 ounces / 500 sheets of = 0.1696
Each sheet of paper weighs 0.1696 ounces
1 ounces / 0.1696 ounces = 5.89623
Thus, you can get five sheets under the 1.0 ounce limit of first class mail with one stamp.
You can ignore the photos below, because they show me trying to stuff 20 sheets into one envelope.