Here's a depiction of how this works:
This is the EX8000 that I use. there are 4 orange lines depicting this as a 4x4 backhaul system. For all routers, it's a multiplier of 433MHz for AC, which is where they get the total. The second band only has 2x2 configuration. This picture depicts how I would configure the extender. For this router, I get around 750 direct connection with about 350 on actual use. If you're using a 2x2, you're looking at 250 or less on the backhaul alone.
[doublepost=1543192135][/doublepost]Orbi differes per model & apparently has stability issues. Both Google & Eero will be limited badly by the 2x2 configuration. The EX8000 is a linear extender & you can actually learn more about wifi by playing with that. IE, lowband & Highband 5GHz has different powers & thus degrades at different rates as you get further from main router. Your main router also plays a role...its needs a 4x4 to take full advantage of the backhaul setup for the EX8000.
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https://www.smallnetbuilder.com/wireless/wireless-reviews/33125-wi-fi-system-roundup
That is a bit older, but covers mesh & should give you an idea of how they are limited.