Complex sentence: One example is Wall Drug, which in 1936 erected billboards advertising "free ice water" and the town of Wall, South Dakota, was essentially built around the 20,000 customers per day those billboards brought in (as of 1981).
Split into simpler sentences: One example is Wall Drug, which in 1936 erected billboards advertising "free ice water". The town of Wall, South Dakota, was essentially built around the many thousands of customers per day those billboards brought in (20,000 in 1981).