Corellia only has three billion people?!?!?
*checks*
LAWL!!!
OMG... looks like its the same in Legends Essential Atlas as well.
So Coruscant has 1 trillion or so. Geonosis has 400 billion. 100 quadrillion sentients in the galaxy with an average of two billion per inhabited member system according to the Legends Star Wars RPG which would be about 50 million inhabited systems. But that's at its peak of the Republic.
Shatterpoint says 1.2 million plus 1/10th that for the Seperatists. So 1.3 million worlds which is a fiftieth of the other number which said systems. I know from the Yuuzhang Vong War that 365 trillion sentients died, which seems like a lot but... it's a fraction of one percent of the total known space population regardless.
With galactic scaling of course things get pretty wonky anyhow. It'd be easier to wrap ones head around if the Core Worlds all got dozens or hundreds of billions in population though.