Max weight is 1 oz. 2000 sheets of 20 lb letter weigh 20 lbs, 1 sheet weighs 0.16 oz. 5 sheets is almost certainly safe, 6 might be pushing it (when you allow for the envelope). If you're using good quality paper (20 lb is practically see through, I prefer 24) then 4 sheets, maybe 5. Viper GTS

I have sent 6 pages of 24 lb paper with just one stamp before and it got there ok. Could have been that it was going about an hour away and the person at the post office actualy figured that in and passed it on.

Pending your paper weight 500 sheets of 17?x22? papers = 20 pounds = 4 reams (500 sheets) of 8?1/2x11 Therefore 1 ream (500 sheets) = 5 lbs 500 sheets / (5 * 16 ounces) = 6.25 sheets. Conclusion: 6 sheets of 20 lbs paper + envelope = 1 stamp