It's estimated that there are between 60,000 to 1000,000 juveniles that are incarcerated in the USA but that number could be higher.
Some Juveniles are incarcerated in Juvenile Detention Centers and some other Juveniles are incarcerated in Adult Jails.
Also according to statistics it's estimated that 200,000 youth enter the adult justice system each and every years so there are a lot of Juvenile offenders because 200,000 youth is a lot.
Well the human population is a lot more than that so it can seem small but that still is a lot of youth being locked up.