For the FreeStyle Libre 2 sensor:

You can take as many glucose readings as you want while the Sensor is being worn. In order to get a 24-hour glycaemic picture you need to scan at least once every 8 hours.

For the FreeStyle Libre 3 sensor:

You do not need to scan your sensor to get glucose readings. The Sensor will stream your glucose data to the FreeStyle Libre 3 app every minute.