I base backup stockage on average usage rates and availability. I just don’t see buying a dozen or more bottles of something just to have on hand. More extra of something than I can use on more than a couple of builds seems excessive. I’d rather put the money into other hobby related stuff (tools, AM, more kits, etc.).
So, yea, I’ll usually keep an extra bottle of my favorite cement (Testor’s Liquid cement) so that I don’t have to worry about running out in the middle of something. Once I crack open the extra, backup bottle, I’ll add a new bottle to my “mental” shopping list for the next time I hit my LHS. I do, however, keep several kinds / brands of cement on hand because some work better at different things than others, but I don’t keep an extra bottle of each one since most only get used for “specialty” jobs.
I’ll usually keep an extra bottle or two of the paint colors that I use a lot, especially basic colors (primary colors or standards like OD, Panzer yellow, etc.) or weathering colors. Rotation of stock on these works about the same as for cement - if I crack open a new bottle because I’ve finished an old one, then I’ll add that color to my shopping list.
One thing that I do keep a large supply of is #11 X-acto blades. I buy those in the 100 ct. packs and don’t hesitate to change blades when needed. (I do keep a set of three different, color-coded #1 handles - one with a new blade, one with an almost new blade and one that the blade is just about done. When the done blade is kaput or I need a really sharp blade, I’ll change the dullest one for a new blade.)
Another high usage consumable is very small drill bits. I usually buy those sizes by the dozen and don’t lose sleep when I break one.
I am a sucker for diorama vegetation materials, though. I’ll buy grass clumps, static grass, ground foam, laser cut plants, etc. any time that I see new ones. A lot of these materials are not kept in regular stock by vendors, so I get them when I see them. I also like to add new colors and variations to my dio-stuff stash.
Now, if I get wind that some favorite hobby consumable is going to be discontinued, I might stock up a bit, but I’m not obsessive or excessive about it. I figure that ultimately I’ll have to find a substitute new product, so I just stock up enough to get me through the transition period (about normal usage of the product for maybe 3-4 projects). I’ll start researching, looking and experimenting then to find a product to do the job.