Paetec and USLEC combined in 2007 for 45,000 medium-sized and large business customers.

Cbeyond has 60k now.

8x8 has 30k now.

That means that you shouldn't be hoping to scale to millions. (And "make it up in volume" as they say!) Start with just 1000 satisfied customers, then 2000. It is all about finding the first 1000 people who will find value in purchasing the services you offer from you at a profitable rate.