Ideally, your book group will have 3-20 members. Three members can make up a good discussion if all are interested in the book. Limiting the group to 20 members or less ensures that each person will have a chance to voice their opinion.
Many book groups choose to meet once a month.
For a more intense book group, consider meeting every other week. For less of a time commitment, try meeting every other month. Once your book group gets started, you will soon learn what works for the majority of the members. If you have access to a large public library system, you can put holds on multiple copies of the book. You could also try to interlibrary loan copies from area libraries.