Only long experience can help you figure this out. All projects should have at least 20% of the developers who have been there for more than 10 years so they have background context to figure out what you will really need. You then need at least 30% of your developers to be intended to be long term employees but they have less than 10 years. In turn that means never more than 50% of your project should be short term contractors. Nothing wrong with short term contractors - they often can write code faster than the long term employees (who end up spending a lot more time in meetings) - but their lack of context means that they can't make those decisions correctly and so need to ask (in turn slowing down the long term employees even more)
If you are on a true green field project - your organization has never done this before good luck. Do the best you can but beware that you will regret a lot. Even if you have those long term employees you will do things you regret - just not as much.
I don’t like working in teams where some people have been there for much longer than everyone else.
It’s very difficult to get opportunities for growth. Most of the challenging work is given to the seniors, because it needs to be done as fast as possible, and it’s faster in the short term for them to do it than it would be for you to do with with their help.
It’s very difficult for anyone else to build credibility with stakeholders. The stakeholders always want a second opinion from the veterans, and don’t trust you to have already sought that opinion before proceeding, if you thought it was necessary to do so (no matter how many times you demonstrate that you do this). Even if the senior agrees with you, the stakeholder’s perception isn’t that you are competent, it’s that you were able to come to the right conclusion only because the senior has helped you.
If you are on a true green field project - your organization has never done this before good luck. Do the best you can but beware that you will regret a lot. Even if you have those long term employees you will do things you regret - just not as much.