The party size increases to 6, and later 8 when you go further into the game.

You can solo (without henchmen) in some areas actually, the entire Pre-Searing, the first part of Post-Searing (as long as you don't take more than 1 mob of Charr), Kryta Gate, Elona... and some others.

The game starts to get difficult once you hit Yak's, (or Lion Arch), and you will start to want real people in your party. In later missions such as Santum Cay, Augury Rock's, Thunderend, etc, it is almost impossible to do with just henchmen. (Even though it is actually easier to do Elona Reach with henchies, because most of the players are idiots)

I really like the character customization and balance in the game. You can get so many cool looking armor/weapons (along with all the dyes, and dye mixing), and the game is far from done when you hit level 20. You can basically pick any profession with another profession and be efficient once you've learned the right skills.