While agents work without human involvement, a developer could run several independent tasks. In practice, the limit is set by their computer: it can usually sustain only one or two heavy sessions.
Every parallel task needs a separate working copy or worktree, its own branch, and isolated project state. Multiple Unity or editor instances quickly consume RAM and CPU. Builds, tests, and automated QA compete for the same resources. Caches, dependencies, screenshots, and artifacts multiply on disk.
Even when an agent needs no attention, the person must monitor local processes, switch between windows, and wait through heavy compilation. A check takes only a few minutes with long pauses between them, yet those pauses cannot be filled with other runs: the laptop is already occupied.