How many days for Yellowstone?
You've gotten a wide range of responses, because it's like asking how many days to see Delaware. Yellowstone is HUGE, and each region of it is different. Fact is, you'll probably wish you had more time, no matter how much time you spend there. So go into it knowing that, and you may be less frustrated. Plan on more than one trip.
Also, don't sweat it about the crowds. It's not crazy crowded in the summer. Just crowded. And again, if you adjust your expectations and go into it with your eyes open, you can even enjoy the crowds. If you went to Grand Central Terminal, you'd expect to see crowds, and with the right attitude about it you could actually enjoy the crowds. Be a part of the "happening." Have fun!
-Speak