It depends. A lot of successful indie games were created by 2-4 people. I think that it is more important to utilize the skills the team have, instead of frenetically trying to compensate what skills are lacking. A team of artists could benefit from making an artful game with simple game mechanism, a team of story writer could deliver a fantastic story telling game and a group of coders could create some demanding and challenging game mechanism/effects.

Extremly good advice, I second that.

On the same train of thought, at some point you need to just DO IT! Start somewhere, iterate and experiment, try, fail and pivot....

It is called Pre-Production and is actually a phase filled with many hours spent experimenting and prototyping, getting stuck on dead ends and throwing away many hours of work. But also finding the things that do work and iterate/refine them, find out what you CAN deliver with your available resources, and learning the new processes needed that will turn your Production phase... well... productive.

You can theorize about what your team can or can't do with N amount of days for the same N amount of days, you will be none the wiser. No one here can give you any number accurate enough to actually use as this number differs a lot between projects, different pipelines and the skills of people involved.

In the end you need to jump in the water... if you are not sure if you can swim yet, don't jump into the deep water yet, use the kids basin.

(or to put it in clearer terms: if you have an idea that would take 100 of hours of work, break it down into its fundamental building blocks, see if you can build that and how long it takes. If its a dialog heavy system, develop the dialog system and refine it. See if it works as well as you thought. If you need 100 of units, build one and see how long it takes you, streamline the design if needed and try again until the build time is manageable).

You would be amazed how small some teams were that achieved quite outstanding results. If you come to a forum unsure about what you can achieve, you clearly lack the skills and expierience yet to achieve the same, but rest assured that anything you do manage to achieve will improve both skills and expierience and will enable you to achieve more with your next try.

You can't expect other people making all the mistakes for you to learn from, some of the failing needs to be done by you ;)