One engineer who takes a product from an empty repository to a running deployment, and is still there when it needs to survive real traffic.
Most projects do not fail in the frontend. They fail in the seams: the API that was designed around one screen, the auth model bolted on in week six, the deploy nobody can reproduce. Hiring separate people for each layer makes those seams somebody else’s problem, which usually means nobody’s.
I build the whole stack, so the seams are the job. Interface, API, schema and infrastructure get designed together, which is the only reliable way to keep them honest with each other.
That covers new products built from nothing and existing ones that need a layer rebuilt. I have taken multiple full-stack products from architecture through production deployment on my own, and I run the infrastructure they sit on.