I am doing a project where I need to send device parameters to the server. I will be using Rasberry Pi for that and flask framework. 
1. I want to know is there any limitation of HTTPS POST requests per second. Also, I will be using PythonAnywhere for server-side and their SQL database. 
Initially, my objective was to send data over the HTTPS channel when the device is in sleep mode. But when the device (ex: car) wakes up I wanted to upgrade the HTTPS to WebSocket and transmit data in realtime. Later came to know PythonAnywhere doesn't support WebSocket.
Apart from answering the first question, can anyone put some light on the second part? I can just increase the number of HTTPS requests when the device is awake (ex: 1 per 60 min in sleep mode and 6 per 60sec when awake), but it will be unnecessary data consumption over the wake period for transmission of the overhead. It will be a consistent channel during the wake period.
PythonAnywhere developer here: from the server side, if you're running on our platform, there's no hard limit on the number of requests you can handle beyond the amount of time your Flask server takes to process each request.  In a free account you would have one worker process handling all of the requests, each one in turn, so if it takes (say) 0.2 seconds to handle a request, your theoretical maximum throughput would be five requests a second.  A paid "Hacker" plan would have two worker processes, and they would both be handling requests, to that would get you up to ten a second.  And you could customize a paid plan and get more worker processes to increase that.
I don't know whether there would be any limits on the RPi side; perhaps someone else will be able to help with that.
Limit of HTTPS request per seconds