There needs to be a core set of tracks which feature every season;
Silverstone, Monaco, Spa, Monza, Hungaroring, Interlagos, Gilles Villeneuve, Melbourne, Rome, Sepang, Singapore and Suzuka.
The additional slots will then be taken up by tracks on rotation;
Valencia/Barcelona, Hockenheim/Nurburgring, India/South Korea, China/Russia, Abu Dhabi/Bahrain, Austin/USA 2 and Turkey/Bulgaria.
That will give us 19 races per season and enough variety to keep the fans and sponsors happy. This will also allow for flexibility should any of the tracks decide they cannot afford to run GP's.