You know how cable internet can slow down when lots of users in your neighborhood are online at the same time? Or heavy cell phone usage in a concentrated area can end up in calls not getting through? That's basically the same thing.

the gps devices we use are all receivers. Satellites send out a clock signal which the receivers decode into a position by what amounts to triangulation. The only thing that would increase the amount of traffic is if it saw more satellites

Yeah, but... GPS is receive-only. Not two way like cell phones.

Not at all the same thing. When you listen to the radio you get the same signal whether you are the only one listening or everyone in town is tuned in. That's basically the same thing.