There are tons of "how many ... does it take to ... " jokes. For example,
Q: How many astronomers does it take to change a light bulb?
A: None, astronomers prefer the dark.
Here, in "Robots-Dreams-Land", a similar riddle might pose a question like "How many wheels does it take to build a robot unicycle?" Thanks to a very kind heads-up from Maurits, we know that one very practical, non-joking, answer is four, as you can see in the video below.
All joking aside, Maurits alerted us to a really inventive LEGO Mindstorms creation posted on YouTube by Vegakade that features a LEGO unicycle using one drive wheel and three other wheels moving in the air to maintain the robots balance.
Similar to two wheeled LEGO/Segway designs, the robot uses optical sensors for balance. However, since this is a unicycle, it required two additional optical sensors plus a center of gravity (COG) mechanism to adjust for tilt along the second axis. That's where the additional wheels come into play.
If we understand the discussion thread on the YouTube post correctly, the wheel on the rear of the post rotates slightly to correct for tilt. But it's movement/momentum also generates some additional effects that are compensated by the larger rotating assembly on the front of the post with two wheels.
LEGO Mindstorms Unicycle Video - the video clip discussion thread includes more details on the design.