Generally the Tern just carries two kids on the back, occasionally we’ve squeezed a third on there. I’ve also ridden with an adult on the back once in a while, which is fun.
A few times we have ridden with the entire family on the Yuba–one parent plus five-year-old on the SoftSpot seat pad, toddler in her seat behind them & the other parent pedaling–but only a few blocks in a relatively flat area. It worked pretty well for a date night with just the two of us on it. Where the bike excels is carrying little kids: two five-year-olds could easily fit on the rack, plus the toddler in her seat behind them. Now that the baby seat is gone, it’s easy to fit three kids on the back.
The Bakfiets has a little bench in the back of the box with harnesses for two kids. We could also have one or two small kids in the front end of the box, sitting on the floor. We’ve used the front end to split the kids up when they were scrapping. Another person could sit on the rear seat that’s mounted on the rack.