I would figure that each person would have 2-3 drinks during the shower. I would average that to about 20 oz. A 2 liter bottle has 67 oz, so you get one 2 liter bottle for every 3 people. I would get equal numbers of regular cola, diet cola, and a lemon-lime or gingerale. I usually get just a bit too much, just in case, and that too much is usually a bottle or two of unusual stuff, orange, grape, root beer or cherry.

In general, I think the older the group is, the more coffee and tea will be drunk, the younger the group, the more soda will be drunk.

I would get Coke, Diet Coke, and Sprite. That's a good variety, since you have coffee and tea. Probably two liters of each would be sufficient, unless you're having a huge crowd.

I would get one bottle of Caffiene free Diet Coke. Some of those moms (to be or nursing) may not want caffiene. Then again they may not want Diet. Oh well, never mind...!