How does one determine how many stamps an envelope requires?

Hi, Momma!

If it's to be mailed within the United States, then you would weigh the envlope and its contents.

Or, is there something more that you're wanting to know, please?

Thanks!

I don't have a scale to weigh envelopes. How much weight requires 2 or 3 stamps?

If it's a business size envelope, the number of stamps needed would be determined by the ounces. One ounce would equal one stamp. Two ounces would be two stamps, etc.

Do you have a scale in your kitchen where you can weigh things in ounces?

I do not. If it helps, it's 21 pieces of paper... Do you think I can get away with 3 stamps?

21 pieces of 8&1/2 x 11 inches?

That's approximately 4oz (3.74oz) including the weight of the (business) envelope. Being a larger envelope that's $1.39 in postage. Slap 4 stamps on it and you should be solid :)