I think Elon has said that the constellation is viable (consistent uptime at reduced density) with 1,000 satellites. So to start they need not 2 million but perhaps 100,000 customers to sign up. Then they can grow the constellation concurrently with the customer base.

2 million customers for high speed / low latency internet for $100 / month seems like a very small ask.

SpaceX projections are that they can win 3-4% of US households, or ~3.6 million households in the US. IMO I would expect the US would be their smallest market worldwide.