Hmmm, I've read the thread and IMHO your decision depends a lot on your budget, your sax 'future-proofing' and your storage space. I have (as an amateur) just 1 (tenor) sax,
Years ago, I kept my previous tenor sax for a year or so as a 'back-up' sax before realising that in that year, I'd never played it. Yes, I had the storage space but it just seemed so sad to me that I had a good, playable tenor sax which I never used. And that someone else would be delighted to play it. So I sold it (via the café) to another member. Not because I needed the money, just because I wanted my 1st sax to be played (rather than just sitting in its case for a year or more).
Some 30 years ago, I discovered that I really enjoyed both the feel and resonant sound of the tenor sax. Over the years, I've had occasional 'try-outs' on borrowed alt, bari and sop saxes. None of then probably long enough to learn to play them. But my strong preference remained for playing tenor sax.
My motivation in learning to play sax has never been about 'learning to play different saxes' but about 'learning to play sax in bands'. For the past 30 years, tenor sax has suited me well in this 'ambition'. I've also never had the inclination (or money!) to 'collect' saxes.
So my (humble) advice is to keep the sax(es) that you enjoy playing now or that you might enjoy playing in the next 1-2 years. And sell the rest.
On maintaining (largely unused) 'back-up saxes', it depends IMHO a lot on how frequently you play, how important your solo contributions are and how much 'back-up' you have I have the benefit of having a wonderful local 'sax repairer' who (on a walk -in basis) quickly 'fixes' any problems 'within 10-15 minutes (while I wait) and usually free of charge. So I very rarely need to have a 'back-up' sax. And even if I ever did, this would involve lugging 2 saxes (usually via public transport) on the off chance that my 'main sax' suddenly developed a 'problem' (which I couldn't myself fix) shortly before a performance. In 30 years, I've never experienced this kind of 'sudden problem' shortly before performances.
Funnily enough, a tenor sax player - who I've played with for very many years in a previous band - did experience a 'sudden problem' a couple of weeks ago. During an initial 'warm-up' rehearsal before the concert, it was clear that many of his 'intended notes' just sounded 'off' and that he was really struggling with his sax Something he immediately picked up on. Neither he nor I could identify the problem. Initially he tried out my sax (as an alternative). But he couldn't 'tune' my sax with his mpc. So he borrowed the tenor sax from an 'interim' (break) sax player which worked fine for him.
So on the subject 'reserve saxes', just think about how often you need them, what 'minor repair' times on your 'main saxes are, whether it's worth taking 2 (main and reserve) saxes along for every performance and what your alterative 'fall-back options' might be. Again, I'm very lucky in being able to phone local sax players with a request like "Hi, I have band performance coming up in (x hours/minutes. My tenor sax has a sudden problem that I can't solve and can't be repaired in time, Can I borrow yours?'
If you don't have access to a 'local speedy repairer' or local 'fall-back' options, then maintaining your own 'back-up' saxes may be a wise decision!