Legion

A Legion was made of 10 cohorts in Rome, like in Camp Jupiter, the most prestigious one was the first, then the second etcetera.

Mentioned

It is not mentioned, in the Percy Jackson & Olympians series that they would be called a legion, because they do not have cohorts. Camp Halfblood differs from Camp Jupiter, because they have cabins, based on who their parent was.

It is brought up, in the newer series The Heroes of Olympus by Reyna and in Camp Jupiter that they were indeed, called a legion and explained what that was.

The Mark of Athena
When Annabeth was escaping Charleston, with the Mark of Athena disc she found, she ran into Reyna , the praetor of New Rome. Reyna told her if she let her go, the legion would be at war with Camp Halfblood, and declare war on their camp.

"Don't underestimeate Camp Halfblood ." Annabeth warned.

"You've never seen the legion at war." Reyna had retorted.