It depends on the type of the startup and industry and can vary dramatically.
Lets start with absolute minimum - if you are building a SAAS product you will need these roles (that might or might not be combined in the same person) that can't be outsourced:
- software engineer
- expert in the product itself or the market and customers for the product
Many engineers start solo because they understand (or think they do) the SAAS product they are building, but it's more common to seek a business co-founder who takes on the visionary role, while also taking care of some administrative tasks (formation, finances, etc.)
For a media/content business you will need someone to produce content (subject matter expert) and if an existing platform can be leveraged (Wordpress, Udemy, etc), you wouldn't need a full-time engineer as a part of the initial team.
First time startup founders are bound to have a huge learning curve when it comes to looking for investors (especially institutional) unless they have a good network to tap into. There is sufficient info online to learn how to put a pitch deck together, so it comes down to networking and ability to sell the idea/team. A founding team with the right skills should be able to build a (SAAS) MVP product before looking for funds. Seed money is often needed to pay for the skills/expertise missing on the founding team.
Obviously, a team for a manufacturing or life science startup would be different from SAAS.
If you're creating a web based application:
That would be the most ideal structure as all those skills are required to be successful (to increase your chances of success anyway).
That being said, you would do with fewer than 3 people if a person can wear multiple hats and is skilled enough to do the multiple roles properly.