It depends on the version of code and what else the device is doing. In general, you have an unlimited number of scripts you can register. However, only a certain number of them will run at the same time. This is configurable using the command:
event manager scheduler script thread class default number NUMBER
Where NUMBER is the number of simultaneous Tcl policies to run. By default, this is one. If you look at the best practices whitepaper at https://supportforums.cisco.com/docs/DOC-12757 that will explain how to test your device to see what is the maximum number of policies it can run at one time.
