Assuming you are interested in running level 80 heroics, these encounters are tuned for 5 level 80 players in decent (blue) gear.

Although some aspects of multiboxing (trivial focused fire) facilitate running these, in my experience multiboxing 5-man heroic content is much less forgiving than when running them with 5 seperate people (PUG issues notwithstanding). Movement and gauntlet scenarios are particularly much more challenging playing multiple toons at the same time than playing as one in a group of separate players.

At my current gear level, I can easily run some heroics with only 4 toons - but not all of them. However, it would have been hard to get to this gear level without running heroics successfully in the first place.

My feeling is that starting heroics with less than 5 toons would be very painful. Of course, if you are willing to pickup or play with other players, running a smaller number of toons and letting someone else tank would completely change the dynamic.

Outside of Northrend, you can roll over a lot pre-WOTLK content, raids excepted, with SOLO level 80 characters. So, it ultimately depends on what your goals are.