1.       How many HKB-2X desktop panels can be supported on a single HMS-4x powerline?  

Ans: 3 to 10 per Powerline depending on cable, the usage of the loudspeaker and how the unit is powered.
If the loudspeaker is meant to be turned all the way up and the units are powered from the Powerline, you can deploy 3 HKB-2X on the line
If loudspeaker is muted and the unit is powered from the Powerline, you can deploy 5 HKB-2X on the line
If the units are locally powered, you can deploy 10 HKB-2X on the line. In this mode, the Powerline is only used for data connection

2.       How many HKB-2X desktop panels can be supported via Ethernet when using local PSUs? 
The data sheet indicates up to 64 units, is this correct? Is there any data limit that we need to be aware of that would reduce the number of desktops / HRM-4X remote stations that could be connected via Ethernet?- How about CAT-5 / 6 cabling limits?   Anything that we should be aware of?

Ans:  64 endpoints is the limit specified for a HelixNet system and is the capability of the main station to maintain internal system connections and bandwith needed for audio routing. An endpoint is an active audio source in the system such as a program audio input (HMS, HRM), interface assigned to a system channel (HLI-2W2, HLI-4W2, HRM SA Output assigned to a channel), a user station (HBP, HKB, HRM, HXII-BP) or LQ ports and accounts (IVC, SIP, Agent-IC). Please note that HRM, HLI-xW2, LQ and the binaural HXII-BP-X5 can be multiple endpoints (resource use is configurable for HRM in version 4.1 and higher).
The system does intentionally not restrict users from exceeding the specified limit. Should the user exceed this limit it is highly recommended to balance the load to different main stations in the system using the resource metering available in the system's CCM web browser (v 4.1 and higher). An overloaded system will start to degrade in performance. Most noticeable at the onset of overload with HMS front panel menu browsing being sluggish. Once this behavior is noticed the user is warned not to load the system any further.
Any unit connected via Ethernet is subject to the normal 100m rule from device to local Ethernet switch.

3.       POE switches.  I’m assuming that the limiting factor is the ability of the POE switch to supply power to the HKB-2X or HRM-4X remote units, the data sheet for the HKB-2X indicates that it needs 13W to power up properly.

 Ans: There are numerous PoE switches on the market and they all have different specs Example a PoE switch might have a limit of 32W max. This 32W reservoir is shared across 4x PoE ports. It will provide more power to one PoE port if it needs it will reducing the power required to other ports  
A HKB-2X will consume 13W if loudspeaker is turned all the way up and loud sound is coming through. If that happens this PoE switch will only be able to support 2x HKB units and will most likely turn the PoE off on the other ports or reset them. But with loudspeaker muted, HKB-2X will consume around 6-7 W. Then the PoE switch will be capable of supporting 4x HKB units  As for recommendations, Netgear and Cisco are good. As long as you pay attention to the maximum power budget for PoE ports AND how the switch handles overpower situations