In the game menu, under "Profile", there is an entry called "Mission Speed". In that menu you can configure the speed with which missions, once the old ones are done, are spawned. To enable the highest speed, you have to use a premium account.

Remember that the maximum number of missions you can have available simultaneously is equal to your largest number of stations of the same type + 1. This number includes both individual and event missions, but not alliance missions, which are counted separately.