Is there a limit to how many users can access/edit content within the system at once?
There are no built in limits but users will start to see performance degridation if too many are on at once. It depends on the hardware CM1 is on and the DB it is using.
This is also why for larger scaled sites we recommend the user of mySQL for the database backend. mySQL can handle larger deployments better and potentially allow for more concurrent users to be connected at one time.
What is the maximum amount of users you have seen so far for a site/what database are they using?
I agreed what Alyce said. I am very curious as well.