Two. If it's just one, you can pick it off and eat the cereal. It might have been an accident. Perhaps one ant veered off its path, got lost and ended up in the cereal. But if there are two, you know it's an orchestrated invasion. No matter how many you remove, there might be others. You can never eat this cereal and feel secure in the knowledge that it contains no ants.
Leonard Cohen (Bird on a Wire): I have tried in my own way to be free
Mrs Hudson (BBC Sherlock): Sherlock! The mess you've made!