The problem with outsourcing, as opposed to remote developers, is that it takes a really good manager and tech lead to make it work. My experience is that you have to write extremely detailed design documents and work specifications in order to get effective results. These generally have to be as detailed as most effective prompts. Once you've written specs that detailed, why do you need outsourced developers and fro…
The entire business model of "outsourced" developers / shops is to overbill people - "we have 4 engineers working on your project" (and also on 5 other projects). Even if the engineers themselves are cooperative, their managers / business owners will resist close cooperation and enforce work at arm's length (e.g. 1x weekly calls). Ask me how I know. I once spent £300k (fortunately not my money) on an outsourced team…
Does it matter how many engineers work on supplier's side?
Supplier is tasked to deliver the project. It is up to them to figure out how many people would they need, and to manage them.