Generalize, don't specialize, that is the key to glassware. Unless you are throwing an elegant affair, you can get by with using highballs and rocks glasses. Almost every cocktail can be made in these two glasses. If you are serving a lot of frozen drinks you can but don't need to use a large wine glass. You should generally have at least 2 glasses per guest on hand. People may change the type of cocktail they are drinking, or they may misplace their glass. Remember that it is not necessary to have a new glass for each drink. The abundance of glasses could create a dishwashing nightmare. If you are having an outdoor event, it is acceptable, and a great idea, to use plastic or disposable glassware.
See our Stocking The Bar section for more information on glassware.