I believe a new jiraCount macro would be the best to implement this feature.
Details:
It would make a lot of sense to add a style=count in the existing jira macro, if all the other parameters of the macro were still valid for it. A lot of them would be, but the fields, fieldNames and maxCount parameters wouldn't work for it. So it makes me think that the macro is really geared towards displaying issues and introducing style=count wouldn't fit well enough. In other words, we would need to indicate in the doc that these fields must not be used when style=count and that's a sign of a wrong design to me.
Thus, IMO a jiraCount macro seems the best option. It would share the id, url and source parameters with the jira macro.
PS: Don't hesitate to send a proposal on the forum to discuss it with others.
