Do you mean editors? You'll contact (potential) reviewers on a case-by-case basis, so it doesn't make much sense to say that a journal "has" reviewers.

Editors: this will depend on your field and on how specific you want your journal to be. If you concentrate on one very narrow specialty, you will be able to work with very few editors. If you want to cover multiple specialties, you will need different editors with different areas of expertise so each submission can be assigned to an expert in the field.

This will depend on your field and on how specific you want your journal to be. If you concentrate on one very narrow specialty, you will be able to work with very few editors. If you want to cover multiple specialties, you will need different editors with different areas of expertise so each submission can be assigned to an expert in the field.

You may be able to learn more by browsing the editorial boards of journals in your field, and noting whether anything is said about specialties of the different editors.