Teams can use up to four drivers per season, but all of them must have an FIA Super Licence.
During a race weekend, teams can make a driver change with the permission of the stewards any time before qualifying. New drivers must use the engine and tyre allocated to the driver he replaces.
Teams can also run up to two additional drivers in each of Friday’s practice sessions although they are still limited to two cars.
Additional drivers also must have an FIA Super Licence, but teams must inform stewards about their plans before the end of initial scrutineering on the Thursday (Wednesday in Monaco).
FIA F1 2017. sporting regulations can be downloaded here.