Looking at Thompson it would appear there are 115 within the UK. http://www.thomsonlocal.com/businessFinder.aspx?phrase=Wimpy&where=United+Kingdom&origld=&ld=&originalWhere=
About 3000
one
well in the whole USA, there is 60,000,000 in and out restaurants.
1 Imperial gallon = 4.54609188 liters = 4546.09188 ml