There is monthly limitation of # of API queries, up to 250K queries per month included at no cost.
What happens if within 1 month, client request over 250K ? Will they get alert notification for this? Or they can't request API till end of that month?
How can they check their usage?
Please refer to Section 6: Throttling Limits in the RDP API Getting Started user guide.
Please also note that each individual API endpoint has its own limits which may be more granular then the broad guideline specified here. For e.g. here are the limits for /data/historical-pricing/v1 as shown in the API Playground Reference tab:
Historical Pricing API Backend 'burst' limit Backend 'rate' limit User 'burst' limit Operation 'rate' limit
GET interday-summaries 2486 2486 50 50
GET intraday-summaries 520 520 20 20
GET events 350 350 25 25
GET single-event 460 460 75 75
POST interday-summaries 150 150 15 15