There is no maximum limit to the number of feeds that can be polled by the Threat Intel app. The app uses the memory allocation assigned to the docker instance to complete polling operations. If you experience issues with polling errors or failures, contact QRadar Support and they can look at increasing the memory allocation for the app. We do not publish articles on how to tune memory allocations, but by default the Threat Intelligence gets a default allocation of 800 MB in QRadar. Support can assist you with any issues you have with polling sources, but ~20 sources should not be a concern if you are not seeing errors.
Hope this helps....
If you have follow-up questions or concerns, let me know.
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