As far as I know there is not limiting when it comes to API calls. Fugiman talked about it here. As far as I know this policy hasn’t changed. What rate are you getting that 503 error at? Once every 15 calls, maybe more maybe less? I have a script that currently grabs the same summery metrics of viewers and channels and sends it off to a data stream; this runs every minute and I just have an error check to make sure I’ve pulled information before I send it off. Not sure if you’re doing this already but it’s a good idea.