1. What fraction of a population has to be living or travelling in space regularly or simultaneously for the whole population to be called a "Spacefaring Civilization" ?

Your suggested numbers are a bit dumb. You need a few more magnitudes in there and less decades.

You also need to clearly distinguish between "regularly" and "simultaneously"

As an example, let's look at airline flight and an analogue "airgoing civilisation":-

Something around 50% of people have NEVER flown, even in countries like the USA. In undeveloped countries, the figure may be closer to 99.99%. Undeveloped countries are also more populous, so they push the averages down.

In countries where people "often" or "regularly" fly, they may count a once-a-year holiday as regular. So if 50% of people fly once a year, for 6 hours, then at any time, 0.5 * 0.25 *1/365 of the population is flying. (!That's ~1/3000 or 0.03%), and we call ourselves a modern society!

I must fly 10 or more times a year, but I'm only in the air for a week, at most (added up, not all at once, dummy!), so that's 2% of my time spent compaining about coach class!

I recognise that space travel may take longer than air travel. You might want to look at similar analyses for the days of sailing ships when journeys took months at a time. Only a tiny fraction of the poulation tavelled "often" and most never sailed across an ocean at all.

Yet, whole countries like yours and mine were populated (or conquered, depending on your point of view) by those travellers (who may have travelled once) and their descendents who never travelled overseas.

So, I think you need to extend your quiz back to percentages like 10e-4 to 10e-6 before you can get meaningful answers. (that's fractions of 10e-6 to 10e-8)

2. What defines the whole of the spacefaring society?

Pass

3. Based on the percentage and population selected, how many people would be actual spacefarers ?

A few billion times a few millionths, give or take 3 orders of magnitude.