I wonder what Project Soul's / Bamco's model for the launch roster and post-release is...
If they are going for a low initial roster count, they may be looking to be more like an improved version of Street Fighter V, with more modes and characters, but not too much more. SFV sales were definitely harmed by their release and initial public reception, but they have kept going on strong especially with DLC. SFV's reviews were decent enough, and perhaps Soul Calibur is aiming for a bit above that.
Even though SC characters are more work, consumers often gravitate to quantity, especially when it comes to DLC, so 6 characters for SF per season has worked well. As others have pointed out, I think having 4 chars for S1 would be okay, especially if it comes with other things and have some of the most requested chars, and later they can expand to 6 chars for S2, or stick with 4 chars but come at a lower price point.