i have around 8 to be on safer side. But I believe 4 slices should be enough in the long term as you can take up missions with longer duration. You can also have few pizza boxes which will help store the pizza received during mission. Regarding the starbits, you can save it to reach 1million and get the millionaire card, or you can purchase packs @10000 starbits/pack or you can withdraw and convert to hive.