Hi everyone,
I have a 5-floor house and I plan to use only Aqara Hub M2, G3, and M3 as my hubs.
• Do I need more than 1 hub on each floor, or will 1 hub every 1–2 floors be enough?
• Can Zigbee devices (like switches and plugs) extend the signal well enough between floors?
• For best stability, should I place the M3 in the center (3rd floor), and then use M2 on the ground floor and G3 on the top floor?
Would love to hear your experiences and recommendations 
No you dont, if you make sure you place matter mains, or zigbee mains devices (must have a neutral wire) bulbs/plugs strips etc that can connect to hub they will extend it and act as satalite routers for the network.
So you could place m3 in the middle and then one plugged in matter and zigbee device compatible with hub (be it a light switch, plug, bulb etc as long as it draws power from a live feed and not a battery and has a neutral it will act as a repeater to build up your zigbee mesh and matter mesh.
So one matter socket and one zigbee socket on each floor and thats it your extended across the whole home once they are connected to m.3 hub.
make sure they are compatible with the hub though
In my opinion I would use one hub per floor, not just to be sure every device is reachable but to get some peace of mind and well structured network.
I am assuming you have some Mesh routers, I also should put a satellite on every floor.
Regarding the question about the wall switches, those with neutral wires can act as repeaters