I usually run one agent at a time in an interactive, pair-programming way. Occasionally (like once a week) I have some task where it makes sense to have one agent run for a long time. Then I'll create a separate jj workspace (equivalent of git worktree) and let it run.

I would probably never run a second agent unless I expected the task to take at least two hours, any more than that and the cost of multitasking for my brain is greater than any benefit, even when there are things that I could theoretically run in parallel, like several hypotheses for fixing a bug.

IIRC Thorsten Ball (Writing an Interpreter in Go, lead engineer on Amp) also said something similar in a podcast – he's a single-tasker, despite some of his coworkers preferring fleets of agents.