Complex sentence: They were very common throughout the 1950s and 1960s and at one time had about 750 restaurants, while only 11 "traditional" and two newer "non-traditional" locations survive in the Mid-Western United States.
Split into simpler sentences: They were very common throughout the 1950s and 1960s and at one time had about 750 restaurants. Today only 11 "traditional" and two newer "non-traditional" locations survive in the Mid-Western United States.