I can understand your concern there over the number of owners/car age ratio. Unfortunately, probably wrongly, it's put me off a couple of purchases in the past.
Ok yes, it could indicate a problematic vehicle, but as Ekona has said above, a lot of people tend to keep vehicles for a relatively short period of time. Could be for numerous reasons - boredom, "been there and done that" or financial to mention but a few potential reasons.
What model is yours? £28k sounds a bit steep for a 2018 Z, even a Nismo.