The function counts a request when it manages to fetch a JSON from the target url. For instance, if fetching the content fails or if there is no element in the JSON that matches the filters, no request will be counted.
The JSON is cached by default for 24 hours. Therefore, until the cache expires, using the same url in other IMPORTJSON functions will retrieve the cached JSON and these operations will not be counted as new requests.
Premium and High Volume plans offer the cacheLifespan option to customize the period during which the data is kept into the cache.
That said, you can always choose to update the data by yourself by pushing “Update content” in the sidebar (Actions screen) or the menu.