When I do "upgrade", I usually hold onto the hold gear as a backup.
I don't have just one of everything, though one of many things.
When I do have a backup, if it's expensive, it's just one thing. My Mac mini with DVI LCD is backed up by an old iBook. My $650 dollar guitar is backed up by a $450 dollar guitar, etc. This is in contrast to having four computers (and all set up) or four guitars laying around in different rooms, which was once the case when I was a hoarder.
But I only have one TV, car, dining room table, stove, fridge, garage, bathroom, etc. My hoarder friend has four TVs and three refrigerators in a very small house with basement.
But some things you usually need three or more of like shoes (dress, sneakers, back up sneakers), and some things it's nice to have ten or more of like socks (4 white pairs, 6 black pairs), batteries (2 Ds, 6 AAs, 2 AAAs), underwear (8 briefs, 2 boxers), toilet paper rolls (at least an unopened package of 6 if not 12) for instance. I did get by for years with one pair of sneakers and one pair of dress shoes and when I was a sporting goods salesman in the shoe department, I would sell tons of sneakers to guys who were replacing their sole worn out sneakers which they would be wearing.
Most of those guys had one other pair such as a pair of dress shoes. If I had a good day with that customer, I would sell them some nice dress shoes and a decent pair of New Balance cross trainers! And in many instances, these shoe hating men/boys would ask if they could have something akin to a Rockport or knockoff which could double as a sneaker and as a dress shoe, so they could cut it down to just one pair of shoes.
This is one place where I found a marked difference between men and women. I do have friends who have three or four pairs, but they are those said Rockport knockoffs, all the same make/model.
Three are two prevailing theories to having "stuff" or inventory. There is the Japanese model of
JIT, or just in time, which means keeping minimum amount necessary and buying fresh inventory and supplies when needed (taught as a staple these days in every MBA school),
and
JIC which means just in case by having lots of everything imaginable. The JIC model destroyed many an American company and this almost uniquely American trait is a cause of stress for people who are in the thick of the serious mental illness of hoarding (a form of OCD). If the barbarian armies were the downfall of Rome, JIC killed the American empire, or dominance they once enjoyed in the Post-War Era through much of the 1970s.