My Maths, or published documents, I forget, indicate the band used is only 250 Mhz (s opposded to 500 Mhz a satellite band usually has), so I reckon something like 16 transponders.
Currently, according to Lyngsat, Shaw Direct has about 3 to 4 HD MPEG2 channels per 8PSK transponder, 7 to 9 SD for MPEG2 QPSK, and 11 for MPEG4 QPSK.
Now, supposing that 3 MPEG4 channels can use the space of 1 MPEG2, and 8PSK will be used for modulation, it could mean around 8 HD channels per transponder, 128 HD channels total, less HD if you include some SD in that mix.
And that is only part one. Part two is they will start converting existing HD channels to start (by putting MPEG4 versions on G1 soon, then turning the MPEG2 version off eventually), and likely some QPSK MPEG2 later on, to MPEG4 8PSK, freeing up that much more space to add channels.