I agree that Houston being the 4th largest city and the 10th largest metro area could possibly support at least 2 EM programs. When you look at the top 15 metro areas according to
http://www.worldfactsandfigures.com/top_50_us_metroareas_asc.php
(in descending order: NYC, LA, Chicago, DC/Balt, Bay Area, Phily, Boston, Detroit, DFW, Houston, Atlanta, Miami, Seattle, Cleveland, Phoenix) and FREIDA for EM programs, there are around 15 in the NYC area, 6 in LA metro, 6 in Chicago metro, 3 in DC/Balt, 2 in the Bay Area, 5 in Phily metro, 3 in Boston metro, 4 in Detroit metro, and 1 in metro areas for Dallas, Houston, Atlanta, Seattle, Cleveland, and Phoenix, but 0 in Miami. Please, correct me if I leave out a program in a suburb of a metro area that I did not account for, but the numbers seem about right (except for Miami not having any). I didn't account for the number of spots in each program though. UT Southwestern (Dallas), Emory (Atlanta), and others have large programs with over 15 spots per class.