I think that the consideration for the number of Black Creatures to support 
Ichorid is very different from the number of Blue Cards to support 
Force of Will, simply because of the rate at which Dredge is able to make these Black Creatures available. Blue is also able to provide a very dependable (the most dependable of all the colors) rate of going through its deck because of the color's natural propensity for card drawing and searching, but this rate pales in comparison to even the weakest Dredger that's being used - 
Golgari Thug (given that 
Darkblast isn't an auto-include for all deck variations).
Having said that, I haven't really given much thought for the 
optimal and minimal number of Black Creatures to support 
Ichorid. I believe I currently have 15 creatures, including the 
Ichorid themselves (I'm running 4x 
Ichorid in the MD), and this number seems to be sufficient for now.
Any additional input on this would be appreciated.
Kind Regards,
jares