At least you have 18 bikes, if one breaks downA goal with my current quiver of mtn bikes has been to have as much cross-compatibility as possible. All three have Boost wheels, F&R, and Shimano 12 spd. The two Santa Cruz bikes are running GX AXS, so one can be spare to the other. The Spot is XTR. All 3 run Shimano brakes, so I only need to keep one bleed kit on hand. They are all different models, tho... Saint 4-piston on the MT, XT 4-piston on the Chameleon, XTR 2-piston on the Spot. I do have a few extra Saint pad sets left over from when those were on my DH bike and I had delusions of racing, so I still have a bunch of spares left over from back then.
Otherwise, I wouldn't mind having a couple of 12 spd chains and maybe a cassette on hand.
Well.. right now I have 6 - the three mtn bikes mentioned above, my Colnago road bike, a GT Grade gravel bike that I'm still trying to figure out what to do with and a 22 year old 20" BMX bike.At least you have 18 bikes, if one breaks down