API queries are subject to two types of limits: there is a limit to the rate at which the API will return information and a limit on how many results can be returned in a single search.
API rate limits depend on which collection you are attempting to access. Most collections are limited to a search burst of 80 requests per minute. If you exceed this limit, your access to the API will typically be blocked for an hour.
The current rate limits are:
Newspapers endpoint
Burst Limit - 20 requests per 1 minute, Block for 5 minutes
Crawl Limit - 20 requests per 10 seconds, Block for 1 hour
Item and resource endpoints
Burst Limit - 40 requests per 10 seconds, Block for 5 minutes
Crawl Limit - 200 requests per 1 minute, Block for 1 hour
Collections, format, and other endpoints
Burst Limit - 20 requests per 10 seconds, Block for 5 minutes
Crawl Limit - 80 requests per 1 minute, Block for 1 hour
Regardless of the rate at which results are returned, searches are limited to 100,000 results at a time.
Please visit the APIs for LoC.gov for more information.