There's probably a rule of grammar that in such a phrase the noun must agree with the quantifier, even when the noun is part of an aggregate. "50 states and territory" doesn't sound right at all. "48 states and 1 territory" sounds stilted. Better to say "48 states and the District of Columbia", but once you include Puerto Rico it gets too wordy.

The original title seems like the best option. Also, D.C. is a territory in a loose sense (part of the U.S. but not a state), just not the same formal category of territory as Puerto Rico. It's an immaterial distinction in a headline such as this.