@2irons 

Hi, welcome to the community.

It is fine to use 5 or 6*Deco PX50 units under the same Mesh system. If you need extra units, another PX50 would be better than Wi-Fi Backhaul-only Deco models.

Have you ever used any powerline adapters before in the house?

If not, I think you could start with a 3-pack Deco PX50 kit and test the network performance first. If everything goes as expected, we could add an extra 3-pack.

There are some frequently asked questions about Deco Powerline Backhaul you might be interested in:

General Questions about the Powerline or PLC Backhaul Feature of Deco

Thank you very much and best regards.