What are the recommended maximum number of Deco's to use in a mesh before performance starts to decrease?

This would be nice to know, for each product line (e.g. X20, X60, ...) and for two "edge" cases:

1) Only the main Deco with Ethernet connection (e.g. to Internet)

2) All the Deco nodes with Ethernet connection to a switch, and in Access-Point mode (for the mesh).

Other configurations could be extrapolated from those two 'extreme' cases (all-WiFi backhaul; all-wired backhaul).

It's not arbitrary, because even with Ethernet backhaul to every node, the "mesh administration" overhead will eventually saturate some part of the system:  either the speed of the backhaul (assuming 1Gb Ethernet switch), the memory limitations on each node, or the CPU performance on each node (at least on the current "Main" Deco node, since it likely has the most "administrative overhead").

Since the Deco lines are sold in 2 & 3 packs, I'm sure the number is higher than 3.  But is it 5?  or 7?

Why might that be useful to know?  Two primary reasons:

A) The number of devices when can be usefully connected increases with the number of nodes (particularly as devices are spread around a house, such as smart thermostats, etc.).

B) In this era of 'home offices' (2?), and students working at home (1 or 2 spaces?), and having 'gaming rooms', that right there is 3 to 5 spaces where one might like to have really great WiFi.  And maybe you'd like to add good coverage when sitting under a tree in the backyard. If your home has thick (or plaster, or brick) interior walls, a single good WiFi node isn't going to cover a lot of rooms with great speeds everywhere.