Galileo isn't second, its 4th or 5th in the market. Depending on how you read the table on this page [1].

>Also wonder how many devices this network can support. I'm sure there is sufficient capacity but I'm just curious how the capacity is calculated!

Satellite navigation systems broadcast signals on a specific frequency. Anyone listening, should be able to pick up the signal. Therefore I assume infinite capacity.

[1]https://en.wikipedia.org/wiki/Satellite_navigation