I have three road bikes and a mountain bike.
I have a CatEye tool bottle in which I carry a spare inner tube, tire leavers, puncture repair kit, tire patch, an inflator with two gas bottles, a quick link, three cable ties a small multi tool, a quick link and a couple of antihistamine tablets.
I switch this between my road bikes and have under saddle tool bag on my mountain bike on which tire sizes and chain size is different to the road bikes, hence different spares.
Additionally I have front and rear lights, discussed above and on each bike speed, cadence sensors and power meter pedals. I switch my head Polar Head Unit between bikes.
On all bikes I carry a 700ml water bottle that is usually filled with an electrolyte solution.
I don’t ride at night.