I'd like to know where that value comes from, is there any scientific basis to it?

When the equation was first proposed, I'd guess they assumed Sol is an "average" solar system, and given that we have Earth, which is by definition Earth-like, and Mars, Venus, and several of the Jovian moons, which could all fit a suitably loose definition of "earth-like", Drake might even have thought he was being conservative.

It seems to be just speculation. There's not much data you can use to guess those numbers with the current scientific knowledge.