Planning a Baby Shower

Well, there aren't any rules about how many to invite. Depends on the size of the "home" or other place where you are having it, how many friends and family they have, how much you plan to spend, etc. A good amount is at least a dozen, besides mother-to-be and host(esses).

Etiquette-wise, technically the shower giver is supposed to be a friend, not a relative, but these days people don't stick by that and probably nobody much cares. You would definitely invite the mother-to-be's mother, sisters, sister-in-laws, close friends and anybody else she deems important (other relatives, close co-workers, church friends, neighbors) and also same for the expectant father. (This being the old fashioned "women only" shower, but if you are more diverse, you can invite the Fathers and other men.)