In answer to the original question; I'd say it depends on how much space you have to keep them. Or, more accurately, how much space you can get away with
I'd say I've got around 300 kits, Including figures - and I only build in
1/35 or bigger! Most have of these have gone into the loft ready for the house move so the stash seems fairly under control now.
Some time ago I started a spreadsheet grouping kits and accessories together in themes. I can say that every kit in my collection has a plan for it and I have every intention of building as many as I can.
I long since came to the conclusion that if I bought a kit, and the accuracy/detail was good enough at the time (with maybe the odd AM pieces thrown in) then it's good enough to build now. I don't have any really ancient kits though, but I do believe there's still a good tune to be eeked out of an old fiddle (I'd love to have the classic
Airfix 1/24 Stuka in the stash). Another example... I still have about eight 1/32 Hasegawa 190s and they are great kits, despite the news that SWS are to do a super detail series of the Focke Wulf butcher bird.
It's not how many kits for me it's the how much time you have to do 'em!
BTW the HK Lanc is another MUST!!!!!
Have fun
Paul