@TomD2
The first link that leads to a blog is pretty much alive and I can open it without any issues - https://www.tp-link.com/en/blog/50/wi-fi-range-extender-why-choose-them-and-how-to-install-them/
There's a topic in there which says:
"
What devices can I connect to a Wi-Fi Range Extender?
Any type of wireless device can be connected: up to 8 clients for Range Extenders that support 2.4Ghzband only, up to 32 clients (16 in 2.4GHz + 16 in 5GHz) for Dual Band models. Extenders with a LAN port also allow the connection of wired devices such as smart TVs and desktop PCs. Where it is necessary to connect multiple wired devices simultaneously (no more than 3/4), it is possible to multiply the available LAN ports by connecting a 5-port Ethernet Switch.
"
In this topic is described how many devices you can connect to a RE in general.
A particular information regarding RE315 can be provided only by the TP-Link admins.
" With the configuration I gave you, should I use the “High Speed Mode” option? "
Yes, if you enable 5GHz radio on the REs, you could use that band explicitly for a wireless backhaul between your router and the REs.
You can test that - thus devices connected to the REs will have a greater bandwidth to your router.