Help understanding functions limits and how to make sure I stay within these limits

Hi!

I am working on a personal project that will use serverless functions extensively, which is a backend part I am still learning. I will use only async functions through the rest api.

One thing I noticed was the limits listed in the documentation.

So, I have a couple of questions:

1. About the Up to 120 concurrent functions, does this mean 120 different functions being called at the time, or 120 invocations of any function running at the same time?
2. I will only use a single async function, does this mean I can have at max 120 invocations of it running in parallel?
3. If I do reach the limit, will there be an error response to easily know when I have to wait, or should I keep a count myself?
4. Also, there seems to be a limit of 600 invocations per minute. Are there common patterns/services I can use to handle this situation?