No limit other than what is sensible given the available CPU power. However you say "simunatenously", so to keep things clear I must say that there is (currenty) no proper support for concurrency. If you want to have different scripts run "simunatenously", you would do just like in standard BlitzMax/Blitz3D (such as having an "Update" function scripts, called each frame).
Hmm just hit me, I want to make sure our defination of "simunatenoously" is the same. By simuatenously, I mean Multi-Threading. In my Nream Engine, one "event" constantly having 2+ script running at the same time. One mite be for movement, and another for acuatl events etc. I want to make sure Brisk can Multi-Thread. Because so far, from what I seen in the demo, both (rain, bouncy) are ran one after the other. And if one script have like "repeat; forever" loop in it, the program will run/stuck in that script forever. I read over the manual multiple times, and cant seem to find such answer/solution. I was wondering, does it support multi-thread?