How many stamps do I need to send a simple condolences card with a simple letter of one page? It's a condolences card for the family of a..?

if it is ordinary size going in domestic mail -- one first class stamp

 Thanks a lot. I don’t think the envelope exceed one once. The condolences card is a medium size card and I put a simple one page letter inside the envelope. Do you think the $0.55 cents will be enough? I’m avoiding trips to the postal office because of this terrible virus. The Stamp attached to my envelope is a forever stamp.

It goes by weight. 55 Cents per ounce, or less. Over that to two ounces it's $1.10.

When you are sending "greeting" style cards through USPS, a typical card can be mailed for the cost of 1 postage stamp. In 2020, a 1-oz First-Class Mail postage stamp costs $0.55 at the Post Office.

If it weighs a bit more, you'll need 2 stamps.