12,000 satellites is the current FCC approved launch figures, I believe. 20Gbps per satellite but I don't know if that's full duplex or how much is divided between terminal/gateway, so lets divide that in half. For simplicity let's say they only sell consumer-grade 100Mbps connections, which are oversubscribed 100x as is typical. Divide the possible number of customers by 3 to account for uneven geographic distribution and other factors.
12000 * 20Gbps / 2 * 100 / 100Mbps / 3 = 40,000,000 customers
A service like that could sell for a lot in rural North America, but maybe not to 40m customers. Let's assume most of those customers are from lower income countries or from places with more competitive ISP environments. Maybe $20/mo average revenue per customer. That's $800m a month. That already sounds compelling to me, before considering the potential for more or higher bandwidth satellites, or for higher margin commercial or government contracts.