Hi Andrew,
You have a great question-thanks for visiting us here at the Disney Parks Moms Panel! The answer to your question is that the number of people that can sit together on an attraction depends on the attraction. For example, the Barnstormer can only fit two people next to each other in the seat. In that case, you would want to make use of the
Rider Switch Service so you can switch off from one child to another. However, for a ride like The Haunted Mansion, the seat can usually accommodate three people. In some cases, like Buzz Lightyear's Space Ranger Spin and Toy Story Midway Mania, you can fit three people in the seat, but only two can interact with the ride, so the person sitting in the middle misses out. You may consider using the Rider Switch Service in this case also.
You can ask the Cast Member at the start of any attraction queue regarding how many people can ride an attraction together; if you need a Rider Switch pass, they can assist you with this as well.
I hope this helps you plan your upcoming Walt Disney World Resort vacation!
Best Wishes,
Amy B.