We don't have a particularly good visualization of holding slot growth potential on SWMH, no.
@IoannesBarbarus can dig-up some statistics we ran to generally assess the situation if he feels so inclined, though.
It all depends upon whether you choose crown focus provinces that have more baronies defined for them in landed_titles than are actually used as part of the history-scripted 'max_settlements' value for the province. This is a lot of them, but some provinces (usually SWMH originals that may have been split from larger vanilla provinces or never had vanilla associations at all) will never cause that event to happen. Most provinces that you'd expect to be able to add some holding slots can-- maybe not up to 7, but more than initially.
The SWMH team was unwilling to either a) use made-up barony names (Paradox literally used a random fantasy place name generator), or b) go to the effort of buffing-up extra, real settlements just for this feature of the DLC.
I'll admit: this upset me a bit at first, but we (EMF) were at least able to come-up with a way to still be able to add holding slots whenever and wherever it is possible (not as simple as it sounds due to need to fully avoid a CTD issue related to provinces having incorrect 'max_settlements'), and this certainly has made the best of things at least. EMF without SWMH, of course, fully supports holding slot growth potential to the full theoretical maximum of 7 for every province on its map (since it's just a slightly souped-up vanilla map).