Since you are asking about EtherNet/IP and how it applies to IO-Link Masters, I recommend posting on Automation Direct's Community forum - there is a broader set of users there that can talk about that.
https://community.automationdirect.comRegarding limits on Explicit Messaging (I am assuming as a Client?), you directly control the rate at which those are sent, so you may need to balance the total number of Server devices vs. the update rate (e.g. you can't have 100 devices every 10ms, for example - just buy a bunch of BX-Ms

). Not sure what specifically you are asking about "real limit" - to you, what "limits" are you concerned about? Number of devices? Number of simultaneous requests? There will always be limits. As I've always said, one person's real time control system is another person's batch system, so please describe what your system looks like and your desired update rates.