Most of the time it's just one for me, though I might get started on the next build 'early', towards the end of the previous one, if I'm waiting for paint, varnish etc. to dry.
Having several projects on the go at any one time has never really bothered me. My builds tend to be part of a much wider project now anyway and as a result finishing one kit is rarely the end result.
i guess it's simply down to what goals you have in the build.
Ive usually got at least two on the go , sometimes up to four or five. I find that i can always do something on one of the others while waiting for paint / glue to dry etc. At the moment ive got the mig 21 for the GB , the yak 25 rv (another GB) in progress and a monogram P61 Black Widow and monogram f101 Voodoo about to be started. Thats not to say something else wont suddenly take my fancy and end up on the bench too!!
Ive got two on the go a long term project when i feel like getting back to it and a build which i can do what i want with it and have more artistic license as the say goes.
Although i do have a project that's a constant aswell which is the house aswell