Hi @InbalK ,
As the row combinations for a given ticket and the potential months in between creation and close date do not exist in your current dataset; there’s not really a default function or a calculated field that can be created that provides this type of count request.
The best work around in this scenario would most likely be to create a second dataset that strictly has dates (by month for your scenario). Then you can join that to your original dataset so that the month fields now exist to be worked with. I would try to refer to a previous community post that is somewhat similar to your case (Count of active events over time - Q&A - Amazon Quick Community).
Hope this helps!