Anyone knows by chance, how many vehicles are registerd on the island ?

I would have as a guess...too many.

Not so many, a lot of vehicles will be registered in BKK or other cities or rather changwats where they have been bought.

too many the roads seem to be solid. i disagree that samui is quite. demends on where you are looking. yes maybe the night clubs get a bit quiet but the roads solid with cars and bikes