I once had a look all the populations from worlds detailed in modern canon books (House Books, TTS, and a few other sources) and the average was a about 1 or 1.5 billion people per world from memory. The problem was that these population numbers can also come from different eras, which further causes problems for working out a base population level.
And what is published is only a small selection of worlds across settled space, so you need to take any such yard stick with a big grain of salt.
A few more sources have been released since then, so that number could have gone up or down.
Interstellar Operations has some numbers for the numbers of planets for each faction in 3025, so you can work out a general number from that, but it would be a very loose number.