We've stayed at all the Epcot area resorts including Swan & Dolphin, and the busses have been different almost every time (for MK & AK anyway). It really depends on time of day & time of year, how busy they are. I've been there where all 5 share a bus, I've been there when it was split YC/BC and S/D/BW. I've read reports of each having their own during peak times.
The DTD busses are just bad in general for all resorts, in my opinion, not just Epcot Area resorts. I waited 45 min. for one at OKW last year, and it was equally as long of a wait to return to OKW. Once we waited almost an hour for return to Kidani. If we return late at night with kids in tow (like from Disney Quest or a movie) we take a cab.
If time is of the essence, take a cab to/from DTD.