Over about 7 years I bought 5 6'6" X 18 3/4 X 2 5/8 all rounders. What I changed was the bottoms. Vee, single to double, reverse Vee, deep six channels, single to flat to vee with double (Tom Hoye's patented Five Stage Vortex Transfer Vehicle). I learned a lot. That's one reason I changed boards back in the day.
Then I got injured and couldn't surf them anymore. Enter the single fin and no new boards for four years. A seven foot single fin was so versatile it filled three of the four places is my working quiver. I would still pull a twinny when it was tiny, but that board did me from four to a very nervous eight foot.
Then I went around Australia and got The Fountain Of Youth in Queensland. That was my introduction to longboard shapes. Between that experience of glide and the boards I saw getting surfed in Ulladulla I realised I had entered a whole new phase in my surfing career and was on completely the wrong boards and would have way more fun on much longer, wider boards getting lots of waves. Between the creep of age, the lack of time via family and work, I was no longer the fit and hard 30 year old who could sit on the inside and rack up the wave count while doing fifty duck dives. I would become the guy out the back getting every set wave he wanted on an annoyingly big board.
So last year I was gifted a board by my Dad (and a couple of hungie from me), sold a couple of boards for another, swapped an unused kite for my first longboard and bought one outright myself. And I suddenly had a completely revamped quiver that should do me for years.
I do eye off performance orientated longboards for some of reefies (think double head high plus waves) but I guess that's what the new gun is for (and the eight footer). I would like to try a mini-simmons, but that's what the Wahoo and Fountain of Youth are for. And having surfed my mates McCoys recently one of them would be cool, they don't paddle quite as well as my other boards so I realistically would probably finish up surfing the recent purchases anyway.
If a ridiculous bargain second handy popped up I might grab one of those listed above (hence my interest in another thread in a McTavish F4), but really I am sorted.
So I change boards out of design interest, injury, age, style of surfing, fitness and wave greed.
I know this is the longboard room, but I reckon most surfers need a three board quiver. A fish, all rounder, gun. In McTavish terms, Pinnacle or Noserider, Evo, and Vuelo or G2.

My current quiver is five boards I surf regularly. And it will do for quite a long time I think until somethin wears out.