Q:

iBeacon region monitoring AND proximity for >20 beacons? 

I have been working on a prototype iOS app utilizing iBeacons to provide location-relevant information to office employees depending on where in the office they are.  The ideal use case is that whenever an employee enters or exits their office, a callback is fired which provides them some information in the form of a notification (it might make a server query to get information first, etc - that sort of thing).  We also want to be able to do this when the app is backgrounded or terminated; fortunately, we already know that beacon region boundary crossings trigger the appropriate CoreLocation callbacks even if the app is backgrounded or suspended. 
From looking around, I understand that broadly, I have two options for how to approach the beacon region monitoring:

Give each iBeacon its own CLBeaconRegion, and monitor for each of these regions independently. 
Monitor for CLBeaconRegions that correspond to multiple iBeacons - for example, each iBeacon has the same UUID and only monitor for a CLBeaconRegion corresponding to that UUID - then try to determine which beacon triggered the boundary crossing using ranging. 

Thus far, I had chosen option #1.  The advantage of this approach is that I get didEnterRegion: and didExitRegion: calls for each individual beacon and immediately know which beacon I have entered/exited.  Also, I only get one enter call and one exit call, which is exactly what I want.  Unfortunately, I just realized that this approach also limits me to 20 beacons (since each beacon gets its own region). 
I'm not as familiar with the exact implementation details of #2, so correct me if I'm wrong.  But it seems that this approach has more drawbacks:

Apple discourages ranging when the app is in the background because the results may not be as accurate. 
The ranging calls fire once every second, while I only want to have "enter/exit" callbacks. 
If the beacons have region overlap, the ranging calls might continually flip which one is "closest", which would further complicate things. 

Basically, I'm wondering if there is a way to utilize option #2, but still have the benefits of option #1 - a quick and easy way to immediately determine which beacon triggered the region change with only one enter or exit callback?  
I hope this question is clear enough.  It's not all entirely clear in my own head, especially how ranging works. 

A:

Option #2 is absolutely more complicated, but you must accept these complications in order to get around the 20 region monitoring limit. 
A few points:

In the background, you only have around 5 seconds of ranging time, which does not give you as much time to average RSSI (signal strength) from each beacon to get a good distance estimate.   So, yes, the estimates will be less accurate.    If you understand this limitation and can live with it for your use case, there is nothing wrong with ranging in the background. 
Yes, you will get multiple ranging calls per beacon after region entry, and you won't get any callbacks on region exit.  You have to write extra code to take care of this.   I have done this by maintaining a NSMutableArray of all the unique beacons (same uuid/major/minor) seen and update it in the ranging callback.  You can then access this array in the region exit callback, so you know which beacons disappeared.  Of course, it is possible that additional beacons were seen after the 5 seconds of background ranging time expires, but your app will never know about them.   With this option, you must accept this limitation. 
While it is true that errors on the distance estimate in ranging may incorrectly tell you which beacon is closest, you have an even worse problem when doing monitoring, because you don't get a distance estimate at all.   If multiple beacons come into monitoring range around the same time, there is no guarantee that the first entered region callback you get will be for the closest beacon.   So if your use case requires taking action based on the closest beacon, then you must do ranging (knowing that there may be error on the distance estimate.) 

A:

The drawback of the second approach is detecting the entry of a particular beacon will be purely based on ranging, that will not work if the application is killed.  The reason is we will get didEnterRegion only once, because we are monitoring only one region with a particular UID.  The next beacon with same UID will not be detected again if the application is terminated or if the background ranging stopped. 
I recommend a combination of the mentioned approaches , 

Use same UID for all the beacons. 
A beacon is uniquely identified using major/minor value that is collected when ranging. 
As mentioned in apple doc, always keep number of monitoring regions below 20 by removing and adding beacons when the user moves from beacon to beacon (better to keep a beacon neighbour relationship graph in the server.) 
Start ranging when entering the region ... and identify major/minor and calculate proximity. 
Stop ranging when exiting the region. 
Find the closest beacon from ranging method (need to skip unknown range beacons). 
Monitor only the neighbours of the closest beacon in a given time. 

When implementing both options, We should consider one fact, An iBeacon will be detected in 200feet distance.  There may be multiple beacons in 200feet range.