We will usually base those guesses on what other people seem to think or what information is available to us. Most people will give a large number, in the tens of thousands. In part I think that has something to do with that we want the world to make sense and be consistent and harmoniously proportioned. And when it isn't, we mentally cover up the discrepancies and pretend that things fit together. Even when they don't.
Most humans work hard on trying to appear normal.
We try to do what seems to be the proper thing to do in the circles we move in. I guess that is some kind of lemming principle. Without examining the hidden assumptions, one simply starts moving around within the frame one is supplied with. And even if one decides to break the norm and do something totally new and different, one is likely to relate that to the perceived norm. That is, one might become self-employed, might design a unique home entertainment system in one's living room, and one might vote for Ross Perot.