I might be dreaming but does the count on DVLA not cap out at 9 previous owners?
ie when you see a car advertised with 9, it is basicly saying 'some number of previous owners - 9 minimum'
With 9 previous owners, current keeper is owner 10. If you buy it, you will be owner 11. DVLA docs will still say '9 previous owners'.
At least I think that's how it works.
For me (regardless of the age or type of car / likely frequency of change) I steer clear above 5 previous owners, and never buy a car from someone that has owned if for less than 2 years.
Just me - I have my reasons that make sense inside my own head (all that matters).
With the e89 being out there since around 2009, 9+ previous owners feels like it changed hands almost every year. That would be more than enough for me to walk away (right or wrong - surface condition & all other things i could check irrespective).
I don't care how good you think you can inspect a car pre-purchase, you will only really know the true condition of a car after owning it for a few months & driven a few thousand miles in a variety of conditions.