I love using just 1 speedlite, I do have more than 1 but the impact and drama you can achieve with just one is great.
As the great John Loengard would say - "If you want something to look interesting, don't light all".
With hundreds of different types of modifiers and light sources the effects you can achieve with just one flash more varied than people think. Just ask Annie L.
Would welcome feed back, comments and opinions on this subject.