PCI requires rogue detection in our point-of-sale, so we hang one in the vicinity of our checklanes (replacing the AP65 currently in place)
We set it up in monitor only mode for full-time WIPS, but it gives us a VC in the store so we can add 10-12 more iAP units later to cover the store with WiFi and extend the WIPS store-wide when they're not servicing clients.
One is not only enough (if it can see the required airspace) its a great start!