I haven't seen this documented anywhere, so posting it here.
As of 2023, the Kindle Unlimited monthly limit appears to be 50 per day, based on how many days remaining in the subscription.
This means that if you have 2 days in your subscription left at the beginning of the month, you would be blocked a bit after 100 books.
When you are blocked, the server returns
Code:
"responseCode": "PROGRAM_BENEFIT_VIOLATION",
"result": "ku-monthly-limit-reached",
"resultCode": "KLU_TOTAL_LIMIT_REACHED",