Tosafot (D"H ka'an bzman hazeh) learns that since we make the laws of Shabbat in sync with what was done in the desert, we therefore consider a r'shut harabim to mean that you need 600,000 people going through that area.
A question that I had was that in reality, there were a lot more than 600,000 people if you include the women and children whom were not included in that count. I've heard that it was really closer to two million people. So why do we say that r'shut harabim means 600,000 people and not a higher number, such as 2 million? One of the answers we came up with is that the only definitive number we have of bnai yisrael in the desert is 600,000. There is no place that says an exact number including women and children. Because of that, we take the one definitive number that we DO have, and therefore come up with that number.
Another question I had that I did not get an answer for is exactly what number do we need to get r'shut harabim. Do we need 600,000 even, or does it have to be the actual number that is listed in the chumash? (I don't have one if front of me but IIRC, it was 603,000)