Enough people should be involved to provide a reasonable breadth and depth of views on the issues that are likely to be important to the people the study will aim to recruit, and who it is intended to benefit.
The right number of people to involve in your research depends on the scope of the study. It should almost always be more than one person. One person is unlikely to be able to represent the needs and perspectives of an entire group of people, and it may be unfair to ask one person to take on that responsibility.
Consider how you will try to involve people who reflect the diversity of backgrounds and experiences you expect your potential research participants to have, and how you will work with people in a way which is as inclusive as possible.
Not everyone involved in a study has to be involved in the same way. A range of possibilities exist, with different people involved with different tasks or some people being members of the project team.
When you are deciding how many people to involve in your research, think about how many people you can involve well.