Yes and No.
More devices does increase the zigbee network but only up to a certain point. Each zigbee device talks to each other via hops.
So if you have your EA-3 and you have 3 light switches all increasing in distance for example, and you tell control4 you want to turn off Light switch 3;
The EA-3 would then find the best possible route to get to Light Switch 3. Since there is only 3 lights and 1 path, EA-3 tells Light Switch 1 to tell Light Switch 2 to tell Light Switch 3 "turn off".
Then Light Switch 3 tells Light Switch 2 to tell Light Switch 1 to tell EA-3 "Light 3 is turned off".
Each hop takes a few ms and therefor the more devices you add (and typically they will start to get further away from the Zigbee Server) will slow the basic functions like lights down.
Now the reason we say more switches are better for a zigbee network, is because it allows for more ROUTES of hops and expands the mesh to a certain point. Meaning the EA-3 can look at all the devices in its mesh and decide what is the fastest and best route to take in order to have the least hops and fastest communication.
Adding zigbee devices to increase the range and distance of the mesh does work but like I said, at some point it will just be too many hops and too much for 1 mesh.
The further away you go, the more hops you need to get there which means you will start to see a slowdown of zigbee as devices get further away.
Hopefully that made sense ^