I definitely agree with @HAWK on this one, don’t start with too many otherwise there’s far more potential for those groups to be quiet and underused and then you get the ghost town syndrome.
when we first launched our Community we had 6 groups: one each for our 4 core products, 1 for announcing and discussion around company news and events and 1 generic, catch-all group for miscellaneous, general conversation (all new members are auto-enrolled into this group). We know have over 50 groups, with some clearly being more active than others. These span some of our more niche products and sub-products., there are geographic user groups around the world, we have industry specific groups (e.g. Not For Profit customers), and there are also private groups for our customers, where our Customer Success team collaborate and work with their accounts in the community.
As with most things Communtiy wise, starting small as opposed to big is better for building momentum. Then you can see where things take you, and shape your strategy towards what you want achieve but based around how you see people behaving. Once you know what typical behaviour is, you’ll have more of an idea how you might want to change or manipulate that behaviour.
Be sure to let us know which direction you end up going with and how it works out.