Enough to add up to the cost of postage. Currently a first class letter of one ounce is 44 cents.
one
One.
One ounce letter, one stamp.
91 cents or 3 stamps of 44 cents ea.
41¢, the same as any 1st-class letter within the U.S.
If it's not excessively heavy or thick, just one.
Postage is based on weight, not distance. A one ounce letter is one stamp.
Any stamp or stamps adding up to 42 cents if the letter is a normal size and does not weigh more than 1 ounce. www.usps.gov
As long as your letter is under an ounce it is just one stamp no matter which US state you are mailing a letter to.
Since both GA and CA are in the US you would only need one 42 cent stamp. Don't be stupid.
A single stamp is required if the value of the postage which is currently 44 cents for a regular letter to be sent first Class is met.
If it just a letter, just one is fine no matter where you send it from and where the ending destination is located. But I think it cannot exceed a certain weight. If you are concerned about it put two stamps on the envelope and mail it out.