I wonder what the practical limits are.
As a senior dev on a greenfield solo project it's too exhausting for me to have two parallel agents (front/back), most of the time they're waiting for me to spec, review or do acceptance test. Feels like sprinting, not something I could do day in and day out.
Might be due to tasks being too fine grained, but assuming larger ones are proportionally longer to spec and review, I don't see more than two (or, okay, three, maybe I'm just slow) being a realistic scenario.
More than that, I think we're firmly in the vibe coding (or maybe spec-driven vibe coding) territory.