One agent at a time goes a long way but quickly becomes a bottleneck. In an agentic development environment each agent gets its own working copy of the repository (a so-called worktree) and a branch of its own. Then the agents can build in parallel without colliding.
We use Orca to keep track of the agents, their terminals and worktrees. The developer's role shifts from writing every line to assigning tasks, reviewing and approving.