If the 5x7 envelope is within the weight range of 1 oz then you can use just one stamp.
If the envelope you have that is 5x7 weighs more than 1 oz then you'll need a second stamp for the correct postage.
If you do just put one stamp on the envelope and it's past the weight range of 1 oz then either it will be returned to you or the post office will deliver it to the person and ask for additional postage.
But I've never had a problem just putting one stamp on a 5x7 envelope as long as it's been 1 oz in weight or less.
The size of envelope 5x7 is the standard size that is used in the United States for mailing letters and invitations so it requires 1 stamp to mail.