If you are going to be hosted at private clubs 2 or 3 is ideal. I almost think 2 is better because it gives the host flexibility to fill the group or keep the group at 3 if the host is concerned about pace of play.
For public golf, four is best. If you have three, you are spinning the roulette wheel on the 4th that might be added. If you do not get a 4th, you are likely going to be waiting most of the day.
If it is more than four, it depends on the people and the leader. The work increases geometrically for every person you add but if the group is right, it can be great.
I have done a couple solo trips and they worked out surprisingly well. Not my first choice but for the right experience I would do it again.