How many books your group wants to read and how often you meet are obviously linked.
Many groups read one book a month and meet on a fixed time and day to discuss it - but there's no hard and fast rule.
Many groups that meet monthly most of the year will mix things up a little bit at times, for example having a holiday party and book exchange during the holidays, or taking a month or two off in the summer. Some choose to meet every other month or less. Some read more than one book for each meeting, some will decide to skip a meeting if they agree to read a particularly lengthy book, some even read different books but on a particular theme.
A key factor in many book clubs' success is picking a consistent time to meet and sticking with it, with occasional (well communicated) exceptions. It's much easier for people to schedule the book club into their calendars if they know that every second Tuesday of the month at 7pm is book club, than it is to lurch from month to month with different plans.
Most new groups starting out will probably find it best to meet monthly and to discuss one book of about 300 pages or less. Having a routine of getting together regularly will help the cohesion of the group, and keeping the books relatively short will help prevent members from getting overwhelmed.
Do you have questions on this topic or advice to give?