Most of my touring has been on my own, but I've also toured with a good friend, and in groups. For LonJOG there were 38 of us.
I think there's definite benefits to each different type independently, and it's more than just the size that's important. 38 made for an enjoyable ride, but we were basically a (happy) peloton passing through the country in our own bubble. We descended on unsuspecting cafes like locusts and rode together most of the day. It became a bit regimented as it needed to be to keep us all together, meaning that we didn't have much free reign to explore.
Solo touring is the opposite extreme, especially when camping. Complete free reign to do pretty much what I want to do, and to change plans is very liberating. I can ride at my own pace, and being alone means that I end up interacting more with the people and places I pass through on tour. The downside can be that in the evening when I'm not riding, it can get a bit boring/lonely. Especially when I've stayed in places like travelodges, it can feel a bit more like being away for work than being on holiday. Also there's no-one else there to remember the ride with you, and remind you of bits of it in conversation for years after, or to look after your bike while you go for a pee.
My favourite tour I've done was the two weeks Ruthie and I cycled round the Highlands and Islands back in 2015 (
link). We both had similar aims for the trip, were of a reasonably similar riding ability, were happy to change the plans as we went along to fit what we felt like, and really enjoyed the whole tour. Two people was still small enough to stop any bubble forming around us so we still had some great interactions with the people (and dogs) we met along the way. Touring a beautiful part of the country and getting good weather for most of it also helped.