Four or Five "extending" devices is about the practical limit since each device will impose a 10-15% bandwidth loss over the network.
So, with 4-5 devices, your network bandwidth is down to about half of the original. If you can get the coverage, fewer devices is better than more in the new "extend" setup.
Keep in mind that the "extend" setup works like the hub and spokes of a wheel.
The AirPort Extreme is the "hub" and each Express resides at the end of a "spoke". The devices at the ends of the spokes communicate directly to the "hub", not to another device at the end of another "spoke".
So, you'll want the AirPort Extreme in a central location so that the Express devices will work most effectively.
What if I run an ethernet cable from the Extreme to the Express and program the Express to extend the network?
This would be the ideal way to extend a wireless network because the Ethernet connection will maintain maximum bandwidth for the network. The Express, which is connected via Ethernet from the AirPort Extreme could then be configured to allow the network to be extended using wireless only.
Careful...if you connect the Express to the Extreme using Ethernet, you will not configure the Express to "extend a wireless network" as you might think. Instead, the Express would be configured to "Create a wireless network using the exact same wireless network name, security and password as the AirPort Extreme network. In addition, the Express would need to be configured in Bridge Mode for this type of setup.
The advantage of using Ethernet is two fold:
1) It provides maximum bandwidth for the network
2) It allows you to locate the AirPort Express exactly where it is needed to provide more coverage. With the "extend" setup using wireless only, the Express is normally located about half the distance from the "hub" to the area that needs more wireless coverage
Post back if you have additional questions and we can help.
This Apple support document may be a handy thing to have around:
When you connect remote AirPorts to a main router using Ethernet, this is called a "Roaming Network". Connecting using wireless only is an "Extended Network"
We have set up our network and it is working very well with stong signal at the exteme perimiter. Set up a Roaming Network with an Extended Network at the perimeters.
Ran ethernet (no more that 100 yards) from router to first AEBS.
Ran ethernet (no more that 100 yards) from first AEBS to second AEBS and AX etc.
Lastly extended the network outside the building in some areas.
It is an open manufacturing plant but we had concrete block walls that diminished a wireless signal. By using a Roaming network and minimal Extended Network, we have the signal strength we need. Your help has been invaluable. Thank you, Bob.