How.many stamps do I need?

First, don't send cash through the mail. You can pay her just fine by sending her a "money order," which is a kind of check. The post office can sell you one, or they are available at the customer service window of many grocery stores. You can also get them at convenience stores. You give the actual money to the clerk. She gives you a paper which looks just like a check. You send the "money order" to your friend in California, and she deposits it in the bank just like a check. When you send money through the mail, it can easily "disappear." People can tell an envelope contains money by smelling or feeling the envelope. Most companies which accept payment through the mail specifically say on their bill "do not send cash." Regarding the stamp, one first class stamp should do the job. The envelope will not arrive "faster" just because you add more stamps.

Yeah you put 2 if you want it to arrive faster