Likely a lot more than 7 billion if you were to look at the population projections between now and the time we could have 300,000 people settled on Mars.
The point though, is that 300,000 people with knowledge acquired from the entirety of history is better than an empty solar system.
The point is that the human race survives.
Yeah, but I don't quite see why I should care about that aside from my default state of caring about the actual people.