I don't have an outdoor theater, but I want one.....But from my reading on this forum...
You should at least go with 2 speakers. so you can get the stereo separation and you can hear the left/right audio like it should be.
If you want a bigger "WOW", add a sub. While it won't shake people off their feet or rattle their butts, it's the added touch that helps.
If you want the "THAT WAS AWESOME" factor, go for 5.1 surround.
It's all in what you want to do. If it's the first one, go with 2 speaker stereo and see how it works for you. Even test it out to before hand to see if that's really the way you want it before everyone shows up. If it looks/sound good to you, then your guests will LOVE IT. You are the biggest critic of yourself, so if you can make yourself happy with the setup, then it's going to WOW everyone else.
For me, I'm waiting for our Sunroom to be built onto the house and I plan on hooking up a proj to the underside of the overhang and have the rear speakers on the corners of the roof. Then run the fronts and sub out in the yard to where my screen would eventually go.... BTW, I've not bought any of the equipment yet, nor have I told the wife...so, don't let her know or she'll kill me....LOL