From experience I know that once a database reaches a certain size the sql limitation of 61 joins can cause problems
I've seen it raised in stack exchang here:Too many tables - MySQL limitation - CiviCRM API errors
And as a git lab issue: https://lab.civicrm.org/dev/core/-/issues/1191
I'm working on a site with currently 70 active custom data tables (50 for activities, 9 for individuals, 2-4 each for other entities) And am concerned that soon we're going to hit this limit.
Adding activities isn't a problem because no activity uses all 70 available data sets.
But will reporting on activities be a problem, because the activity data sets and contact data sets can all be included by default in the report templates. Or are activity reports handled differently now?