I submitted a case to splunk support regarding this issue. They logged a bug issue SPL-37605 and stated it will be fixed in an upcoming release. They didn't say which release but they now know about it.
I'm seeing the same behavior. I have about 50 collections, but the UI only displays 24. I verified that Splunk is still collecting data from the hosts and they are listed in wmi.conf.
This is probably a bug, if true.
There should be no limit. Can you check the total number of stanzas across all wmi.conf files under etc\ ? Let's make sure that those inputs were actually created in the first place.