Kartra's inbound API system has a hardcoded limit of 20 API calls per second, per App. That means that none of the individual Apps within your Kartra account should send more than 20 API calls per second to Kartra.

If the API call limit is exceeded, the system will return a "Too many requests" error message (429 error), upon which you should wait 1 second and try the instruction again.

Implementing a queue system that throttles outbound API calls from your App to a maximum rate of 20 calls per second is highly recommended. That way, if your App goes through a burst of activity, it will queue the outbound calls and process them in a first-in-first-out sequential order, so they never go beyond the limit. It's better to hold them back for a second or two than to get them error'ed out!