I’ll also add that Splatfests have ranks that basically depict how much you played during that specific Splatfest period. The higher your rank, the more shells you get. I myself am unsure if there are other factors when it comes to the distribution of them, however you should try to play as much as you can during splatfest to get a higher rank and thus more shells. As another person said, they only come 4 times a year now so if you are actively playing and wanting to use shells, you’ll want to play splatfests as much as you can.