I have a little experience - non trader just in it for s***s and giggles and i've been up to 14 cars owned at once, though that was a few years back. I have settled down to 5 keepers and one or two in and out ad-hoc now. I can also keep everything at home, so I don't pay for storage.
First off - storage. It costs a LOT of money for space to keep a lot of cars and parts. When you're not bringing any money in from it like a trader does (or, very little anyway) it's a huge outlay/financial strain. It ended up in me running up a 4-figure bill in lockup rental. Gets pretty stressful. Lockup garages are around £50/m at best, each, and a unit of that size you're waaay into 4-figures a month plus rates and utilities.
Second up - time. Pretty obvious. You end up splitting out your time and it feels like you make minimal progress.
Also, cost to run them all. All the parts, MOT's, Road tax (I currently shell out £72/m for 3 cars) and Insurance is murder (remember you can only use NCB once). I had a trade policy 2003-2017 but that was getting cripplingly expensive and I had to give it up, couldnt justify the cost.
In the end it just hit a point where i'd had it, and something had to give. I cherry picked about 5% of the best parts from my stash (mostly 21 Turbo and R9/11 Turbo stuff) and the rest got scrapped off. I couldn't even be there when it was cleared out, I just left the doors open and drove away.
I also scrapped a R19TD, a 21 Turbo shell, another 21 Turbo shell (crashed), a Calibra, I gave away an old Scimitar that i'd inherited with one of the lockups. I gave away (for scrap) 6 21 Turbo bottom ends in various states, several old heads...the list goes on. It just hit the point where enough was enough.
I sold the 11 Turbo, the other 2 were scrapped.
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Sold this too
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Scrapped
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Scrapped
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All scrapped save the 9, which i still have
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Scrapped all of this
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Scrapped
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I'd say the pics about are around 50% of the total I threw in the bin. In my head it's like a switch flipped, and my stash went from being a collection of precious things to a ball and chain.