They try 12 for a few seasons to have one fewer NEL on a 13-leg Race; they tried 10 teams on the Family Edition, which lead to too many NELs on a 13-leg Race; then they tried 11 teams one season (13) I believe with 11 legs, and then settled on the current format of 11 teams and 12 legs ever since.
From a production perspective, 11 seems to work. Can't really explain why much more than that.