Now your question is better understood. You are asking about the motherboard that supports the Xeon processors, and wanting to know the maximum number of cores that it theoretically could support rather than the maximum number of cores that HP officially has listed in its QuickSpecs documentation of that for this workstation. It is true that HP has added HP-certified processors to workstations over the full course of the workstation's manufacturing, and those show up in the QuickSpecs as new ones are added. Sometimes it takes a BIOS upgrade for those to work.
The thing is.... this is a user's forum. We are not HP. To get an answer to your question about HP's future plans please contact HP workstation engineers directly. They don't come here but you probably can call them. We have no idea what their plans may be. Please let us know what you find out...
The answer from the current Z4 G4 QuickSpecs from July of this year is 18 cores in a Xeon. See page 5 of QuickSpecs v24 from 7/18/20 I downloaded and highlighted for you, attached below.