Your API key is subject to monthly validation quotas and rate limits expressed in requests per second.
The rate limit applies to how many requests you can send per second, not how many are in-flight at once. Since validations can take several seconds to complete, you can continue sending new requests at your rate limit while previous requests are still processing.
If you exceed your monthly quota, requests will return a 400 error.
If you exceed your rate limit, requests will return a 429 error.
To find out the quota and rate limits that apply to your company, please contact your account manager.