I've run solo campaigns in person from time to time, but it has always been a side adventure connected to a larger campaign.
It is certainly possible to run a solo only adventure, but perhaps more challenging, as the Dungeon Master becomes the only source of character interaction the player gets.
JB used to have many solo Role-playing Games here but since he stop running FE and World of Medieval I do n't see them anymore. They were a good way to build up character experience before joining a higher level party.
Solo campaigns are always a possibility in my games. There is a higher reward from such games but there is a much higher risk in them too. I have no issues running a game with one person.
It only takes two to tango so of course you can have a great game. The player in the game could even roleplay more than one character but not more than 3 otherwise it becomes sloppy.
This works out alright until the player has to leave and the game is lost. At least when you have more than one player the game can still continue.
We interrupt Can One Player And A Game Master Carry A Game? to share wisdom from past years:
Today is: 18th August (GMT), in history on the 18th of August, 1774 AD the following birth happened:
Meriwether Lewis: Charlottsville, Virginia -- Captain (Lewis & Clark Expedition)