How many spelling mistakes should a local greasy spoon or kebab house menu have?

None. I critique menus when I go out to eat. Any place who wants to stay in business should be able to spell. I totally agree with you. If they can't be bothered to put out a correct menu, I highly doubt how they stay in business. It's a major pet peeve of mine. I made sure my menus were always perfect. I didn't want customers thinking an idiot was running the place.