It's always the wife or the husband asking that question I notice.
Here's the thing, if your desire to buy new things is not harming anyone, well, there's nothing wrong with it in my opinion and if it helps you stay smoke free, well, there you go. Etc. But, some folks like collecting different SORTS of setups, and others like backups of what they really enjoy.
I did some stockpiling early on and I've also won things and bought the wrong kinds of things in my quest for a perfect vape and etc., (which I've usually given away or donated.)
Now that I have many things I like, it takes something I really find exceptional or just COVET to make me pull out that checkbook. I have nic for life, flavors are a HUGE pull as there are so many of them.
I would discuss with your wife not so much "how many mods are too many" but how much she is comfortable with you spending (and if she has hobbies, too, well it's certainly fair to COMPARE them, I mean my husband has a CEMENT MIXER for instance) and then judiciously decide how you want to spend that money.
I have seen (and experienced watching others) who tend to be quite happy with their setups, slow down on the purchasing and really focus on what next FEATURE they want, not so much "how many mods."
I also think there is a place for rotation and stuff, I don't use ALL my mods at once (although I usually have around 12 active setups at one time) but I do rotate them in and out, as older gear can have new life (for me) and feel "new" again to me, and that sensation is enjoyable (to me).
I do need to have a bit of a fire sale, and by that rid myself of certain items that I know I won't be likely to vape at all as I have other setups that supersede them, although I will say, I do that with a certain amount of "judiciousness" nowadays, because some of what I love is older, expensive, and hard to obtain. Other things, not so much, etc.
Break it down to a hard number would be my advice (excluding stuff that needs to be replaced in an ongoing way like coils, eliquid and batteries) and you'll probably both be happier about it.
Too many mods is kind of nebulous. "X amount of dollars per month/year" is not, and you'll be less likely to impulse buy, more willing to wait for sales on great items, etc., all that good stuff.
Anna