It depends on how many other types of drinks you are serving. Assuming a full bar (wine, beer, mixed drinks, etc) rum is typically the second most popular spirit, vodka is the first, so for 100 people two bottles (26oz each) should be more than enough.
On the other hand if you are doing a featured cocktail like a mojito or something similar with rum, or are serving less of a variety of drinks i would increase the quantity.