To my knowledge, true limpets (taxonomic subclass Patellogastropoda) don't have a hole on top of their shell at all, but keyhole limpets (taxonomic subclass Vetigastropoda, order Lepetellida, superfamily Fissurelloidea) all have one for respiration.
For some examples of true limpets:
This is one of those spottings that made me smile, because this cluster of limpets looked like a town meeting, and it's something I'd never observed before. Cellana tramoserica, commonly known as "variegated limpet" and "common limpet", is a species of marine gastropod mollusc (sea snail) in the...
www.projectnoah.org
Limpets
australian.museum
en.wikipedia.org