This should help.....From UAL...

"We announced Tuesday an order to purchase 25 new Boeing 737-700 aircraft. The order is in addition to the previously announced order of 40 737-700s. We will take delivery of the aircraft beginning at the end of 2017.

The new 737-700 aircraft will enable us to continue utilizing larger, more efficient aircraft as we reduce the size of our 50-seat regional fleet. We expect to have fewer than 100 aircraft in our 50-seat fleet by the end of 2019."

UAL had 256- 50-seat aircraft at the end of 2015.