You can send up to 1 ounce via USPS in a regular envelope with one stamp. This works out to around 5 sheets of “regular” (I used 20 lb 8.5×11) in a #10 (4 1/8 x 9 1/2, “business”) envelope. That sixth sheet put it over the ounce mark. How do I know this? I use an Oxo food scale that I absolutely LOVE! See the USPS Postage Price Calculator for more info.
How many sheets of normal paper can I send in an envelope with just one stamp?