I am pretty new to MongoDB. I am in a scenario where it is possible for a system to invoke functions simultaneously many time.
I have gone through mongoDB Atlas function documentation and didn't find anything which speaks about scalability or concurrency issues.
Can a single function be invoked multiple times in parallel? for example: Three different request trying to invoke same function will all three request be handled one by one or in parallel.
You can call the functions concurrently, provided the workload adheres to the App Services' limitation of 5000 concurrent requests. So, to address your point: if 3 different services try to invoke the same function at a time, they will be handled in parallel.
Additionally, you can use HTTPS Endpoints to expose a Function and trigger it through an endpoint call.