I don't go on these anymore and I am early 30s. Just didn't find them fun for what it costs, rather use my vacation time for trips that I deem 'fun'.

But if I did, no more than 6.

4-5 That way you can fit in one car but also enough people to split up and do a couple of different activities if you have varying interests.

4-6

4 is best. Less opinions to deal with. Less hassle. Less waiting for someone to get ready. Someone is always late, always complaining, always something to deal with. And it's noticeable when it's more than 4.

I have gone on trips of 2-5 and those have all worked well. But I don’t have more friends than that!