I am not aware of any site that will give you the number of people per ride. But if you have any specific rides I am sure that someone here can help. When you get to the front of the line, the CM will ask how many are in your party and will do their best to get you all together.
The toughest place for riding together that I can think of is Dumbo. Each elephant can really only hold 1 adult and 1 child. 2 adults would really be a squeeze. On Space Mountain, it is one behind the other - so if you have someone who really likes to sit next to and hold onto someone on the faster rides that would be a problem. Other than that, most everything else can sit 3-6 people across and the CM will tell you which seats to take.
But like I said, if you can think of any specific ride that you are concerned about, please let us know and we will be happy to tell you how the sitting is (you can bet someone here will know).
Hope this helps.