For a SASE that is a #10 envelope (4.125" x 9.5") with 2 6"x4" photos a single US forever stamp works fine.

For your request, if you send a #10 envelope with LOR, 6"x4" photo, and SASE, one US forever stamp also works fine.

For each ounce, you'll need one stamp. One ounce is roughly four sheets of printer paper folded up in a #10 envelope. If you are using a #10 envelope and think your request weighs more than this, it is better to be on the safe side and add another stamp.

It depends on the size of the envelope and if it would be considered a "package" with the cardboard. Some post offices say yes, some say no. Large envelopes (say a 9x12) can often cost upwards of $1.40 - $2.32.
This calculator should help http://postcalc.usps.com - You may also want to take it to your local post office to be sure

Within the USA, how many stamps should be put on the self-addressed envelopes? I'm guessing the envelope with a letter, photo and sheet of stiff cardboard will weigh between 2 and 3 ounces. I don't want to have not enough stamps, but don't want to waste any either.

Requesting autographs from celebs is new to me.