More than 15-18 members make it difficult for all to participate in discussions, but it can be done. Eight couples 16 individual members might be a good size for a couples group. Some groups take off the summer or take off a busy month such as December. Others plan a non-reading activity in some months, such as a picnic or cookie exchange. In this way, members can plan ahead to leave that time free.
Most discussion groups select one person for each meeting to facilitate the discussion.
This leader may provide background information on the author and the book and will work to keep the discussion on track, to head off digressions, to be sure that everyone gets a chance to participate, and to ensure that all the important elements of the book are mentioned. You will have to decide whether every member must take a turn at being leader, or whether those unwillingly to do so can opt out. Using two co-leaders is always a possibility to ease the pressure. This leader is usually paid. You ll probably want to keep a mailing list of names and addresses and make this available to all members.