When thinking of supporting cross-wiki notifications we can imagine different kind of cross-wiki patterns (e.g., user mainly participating in a project across a small group of languages, participating on many different projects in the same language, etc.). Currently, we don't know how often those patterns are happening.
To get a better picture we can measure the number of users with unread notifications from different wikis. In the form of "X users with unread notifications from Y different wikis" for different values of Y (example scale: 1 wiki, 2 wikis, 3-5 wikis, 5-10 wikis, 10-20 wikis, 20+ wikis).
We can adjust the queries to group wikis by project (e.g., all "Wikipedias"),or limit them to those notifications generated in the last couple months.