Here is a basic assortment of glasses to keep at home for casual entertaining.

For mixed drinks and cocktails:
4 to 6 cocktail (martini) glasses
4 to 6 highball glasses
4 to 6 collins glasses
4 to 8 single or double old-fashioned glasses
4 to 6 brandy snifters

For wine:
4 to 6 red wine plus 4 to 6 white wineglasses
  or 8 to 12 all-purpose wineglasses
4 to 6 Champagne flutes or tulips

For beer:
4 to 6 beer mugs or pilsners