The number is just how many subwoofers one have, so I have profundus Y-8 (for 8 subwoofers). He only sells them in pair and have always recommended at least 4 subwoofers (even so that he recommend 4 of the smaller X sub over 2 Y sub 10" vs 12").
The profundus Y subwoofer I have are 12", 177 liter subwoofers with a sensitivity at around 96dB and -3dB at 14Hz.