Nah, you're doing it just fine <looks at 20some ArM5e books and 20some ArM4e books on his own shelf>
The thing is, you /CAN/ use a whole bunch of supplements, if you want. They're there, available.
But none are really NEEDED. The core book is sufficient for a big ol' campaign!
The closest thing to "needed," IMHO, is: if any of the players want PC's from one of the 4 Mystery-driven Houses, and the SG and/or the table agrees they should have access to their House Mysteries (for alternate, non-Hermetic routes to magical power); then you want the relevant (HoH:MC) book.
Of course, that leads to players of the other 8 Houses wanting THEIR HoH volumes (HoH:TL & HoH:S)...
I'd put almost every game in this category. Multiple core books is a rare thing, splats are generally exceedingly optional. Sticking to ones I particularly like there's the Fudge 10th Anniversary Edition, Hollow Earth Expedition, Microscope, and Reign.
Talislanta 4th Edition, aka The Big Blue Book. Yes there were other books to go along with it but you could easily run the game with only the core book. I am actually a little surprised no one else mentioned it.