I have just completed setup of our Orbi Pro system with 1 Router + 5 Satellites, providing coverage across 4 floors of our entire church building. Before we went this route, I had searched for "maximum satellites" as there are reports of 4,5,6 satellites working despite the official word from Netgear still being only 3 satellites. True enough, it can be done and the community recommendation is when connecting your 4th satellite and 5th etc., shut down all satellites and power off/on your main Orbi router. Then add 4th satellite, power off satellite, power off/on router again, add 5th satellite, power off satellite, repeat. Finally turn on main router, and all satellites!
My 5 satellites are working in star topology, combination of wired and wireless. Initially one connected as daisy-chain (so that feature works), but then later it showed up as star topology. I have confidence to add a 6th satellite if we ever deem it necessary to widen coverage or increase throughput in certain areas. So I guess no one really knows the maximum as I haven't heard of someone trying to add a 7th and say it can't be done.
I am writing this to help others who may think of using Orbi Pros in a larger setup like this. Though the satellites are not cheap, the fact they come with mounting brackets, ease of wired or wireless installation, guest network, and ability to self-install without vendor makes it quite a good deal.