Here are the figures for the 737 NG’s; it is a matter for debate whether they are variants, derivatives or new aircraft
From
http://www.boeing.com/news/releases/...e_980219a.html
"This is a proud day for all of the employees who've worked so hard over the past few years on the Next-Generation 737-700 program," said Jack Gucker, vice president -- 737/757 Derivatives. "Together with the JAA and FAA, we have put the 737-700 through one of the most comprehensive and rigorous flight-testing and certification processes in history. This certification is a significant validation of the airplane's safety, reliability, performance and readiness to enter passenger service. It allows us to proceed with our first deliveries to European airlines -- the first two of which are to Maersk and Germania."
More than 20,000 engineering laboratory and airplane flight tests were completed before certification was awarded. The 128-to-149-seat 737-700 began its nine-month flight-test program Feb. 9, 1997. The total certification flight-testing effort involved four 737-700 airplanes, which completed nearly 1,600 flights; 2,220 hours of ground testing and 2,000 hours of flight testing.
In addition to the 737-700, the Next-Generation 737 airplane family also includes the 160-to-189-passenger 737-800; the 110-to-132-passenger 737-600; and the recently launched 177-to-189-passenger 737-900.
Each of these airplanes will participate in separate flight-testing and certification programs. When completed, the total Next-Generation 737 flight-test program will have comprised 12 airplanes, including the four 737-700s, three 737-800s, three 737-600s and two 737-900s. When completed, the flight-test program will have totaled more than 3,500 in-flight test hours.