I only have 4... plus borrowed stretchies for when they were small (ish!) I've just given away most of my baby stuff (next lot to go in next car trip to see them!) and have decided I still need all 4
OSBH blue ringsling - first one I was given, brilliant for up&down walkers
Hand made MT - this is the one I'd get rid of if I had to, but when we go hiking, its good to have this option with J
Didy wool blend 7 - this is "J's" sling - he prefers this one, and usually on my front, good for using with B in emergencies!
Wrap tai (girasol conversion) - this is "B's" sling (though I use it for J for back carries when B is on his bike, my current favourite)
If I only had to have 1, it would be an impossible decision between the conversion & the wrap - if I'm carrying both, I use these two, both work with either child - Matthew would never use the wrap, but will happily use the others