I'd say that 25,000 is the absolute upper limit for the planning (and probably slightly higher than where Universal will end up).
With the current UOR heading for 9,000 rooms; it's feasible to think that the south resort will be able to handle somewhere north of that number due to the OCCC proximity.
But how much more?
I think 11,000 rooms is the minimum for a 2 dry park south resort near OCCC, with the upper limit somewhere around 15,000.
That gets you somewhere around 20,000 to 24,000 hotel rooms which is probably perfect for 4 dry parks that are seeing 10+ million visitors each. You can keep 90% capacity and healthy, profitable pricing.
Only if attendance is measurably higher (i.e. each park is seeing 12+ million attendance) should they consider going to 25,000+ rooms.
At that point, it's worth revisiting this consideration and buying up cheaper <$100 room rate hotels (the parcels in green on Universal Blvd. in image below) that are getting squeezed and renovating them into Loews-branded hotels: