The number of batteries is based on a number of things and the distance any particular flight can be is limited to 10 km by DJI. This limit will be raised in the next firmware release. Allegedly. The and of a battery tends to coincide well with when you start hitting 10km which is what likely triggered the Go Home. We don't start the next leg if the end of that leg will put the total flight over 10km. We knew this was going to be changing so we didn't invest a lot of time into making it better.
If you use one of the battery timer settings, the estimation is pretty accurate. If you use the battery limited setting it will go until the battery runs out or the segment that breaks the 10km limit is hit.
This will continue to be improved upon as DJI changes things.