Your API is limited to the API Plan you have purchased. However whichever plan you are on, to ensure calls to API are under fair usage policy, these are certain limits you need to remember when coding. They are as follows:
Maximum of 20 API Calls per minute
Maximum of 300 API Calls per Hour
Maximum of 10 API connections at a time.
Pagination is set to 100 records at a time. Please note that if 100 records are fetched, it will count towards 100 API credits being used.
If you exceed any of the limits listed above, you will get back an HTTP Status Code of 429. Just waiting it out for a few minutes/hours, will fix the issue.