Where does your plan live when multiple agents work the same repo?

Building in this area, asking because I might have the wrong model of it. Everything I build assumes there’s a written plan somewhere (a tasks file, Linear, GitHub issues) that agents pull work from. But I keep hearing from people whose real process is closer to “I tell each agent what to do in its own terminal and hold the rest in my head.” If you run several agents: is there a shared written plan, or is the coordination happening in your head? And at what point did you need one? two agents, five, a second person?