When I started looking for my Z4MC, I was concerned with previous owners. However, it soon became apparent that most had 4,5,6+ owners and was normal. My reason for originally being concerned was not the worry of multiple owners increasing the chances of bad owners but that the car may of been a dog and passed on because of inherent issues. I now believe it's more of a sports car thing, a get it out of your system and sell on car that most high end performance cars go through. I do think z4m's regularly changing hands will slow now they've started to appreciate.
You can never be sure about how an owner has treated the car, whether there's 1 or 10 on the log book. Loads of BS is written in adverts, e.g "1 careful lady owner, fsh". This only proves it was registered to a female person from new and serviced. It could of been driven by a clueless idiot son, daughter, husband or the registered owner and ragged to an inch of its life constantly. It could if been pampered, warmed up before very drive, cooled afterwards etc. Same goes for 4,5,6 owners, maybe, maybe not.?
If two cars were equal condition and history and one had 1 owner and the other had 9, I'd go for the one owner. But would take the multiple owner in better condition with better history every time.