The math works out to roughly 1/300.
So if 100 players earn about 3 non-premium containers each. 1 out of every hundred players would get a ship.
That will likely be many thousands of players that do that world wide. So if 100,000 players worldwide each get 3 containers, WG will have given out roughly 1,000 ships.
Usually there's a mission chain that shows up around Black Friday stuff time that allows you to earn some non-premium containers.