Server & API load testing

I have custom API included in a website which creates new UID on new unique user visit like Google Analytics and sends the UID data to the backend server(NodeJs) for computation.

1. I need to check concurrent users and max the limit of users can be created/handled per current cloud config.

2. Also, need to check, is there any limit on API creating and sending users data. The API is on CDN(fastly)

Please suggest some testing tools, to check for above scenario.