For V1: If you are using API version 1(V1) then the number of API calls per hour is restricted to 1000 across all plans. If your API request is received after the limit has been reached, freshservice will give you an error response. The "retry-after" value in the response header will tell you how long you need to wait before you send another API request.
If you are using API Version 2 (V2) then the rate limit is different for each plan as below:
Limit (Per Hour)