Sadly, there's no number you can assign that makes much sense.
If you assign a big number, like a billion, than you have to ask why a fledgling colony of starry-eyed pioneers has a whole billion (or two) people in it, and only stops being a backwater when it has ten billion.
If you assign a smaller number, like 100 million, then you have to ask why all the starting planets have so few people on them, and why an ecumenopolis or ringworld can only hold such an obscenely small number of people - a hundred billion or so at most. An entire planetary surface covered in even mild urban sprawl ought to hold trillions, and orders of magnitude more if it's arcologies and the like. The numbers for a ringworld are dizzying.
Some players get around this with some kind of headcannon about the pops representing some kind of logarithmic scale (IE, each pop on a planet represents more than one before it) but this creates even worse problems and is, in my opinion, very silly. If the first pop is a million people and the tenth is a billion, then how come they produce the same amount when the work a district? How what happens to all those people when you resettle a pop from a dense planet to sparse one? Do millions of people pop into existence when you resettle from a sprase planet to your crowded capitol? The whole thing falls apart under the slightest scrutiny.
Best not to think too hard about it.