How many different original U.S. air dates appear where the U.S. viewers were 2.5 million?
create table table_25246990_5 (original_us_air_date varchar, us_viewers__millions_ varchar, PRIMARY KEY (original_us_air_date))
select count(original_us_air_date) from table_25246990_5 where us_viewers__millions_ = "2.5"