Each request consumes one row + number of results * price of each result. For price of each result, please look it up in documentation for each function.
Most of requests will cost one row for the request itself + number of rows of returned results for the said request.
For example: your request returns 1,000 referring domains when you are working with 'refdomains' API call - https://ahrefs.com/api/documentation/refdomains
The total number of rows consumed will be:
1 row (the price of the request itself) + 1 row (the price of 1 result returned) * 1,000 (number of results returned) = 1,001 rows
Do note that "Each result" means not each specific object of the API response, but the whole response returned for the target domain/URL you request the data for.