According to the Bureau of Justice Statistics, US jails in 2022 held 1,900 juveniles, of whom 1,600 were being held as adults.
In 2021, US jails held 2,000 juveniles, of whom 1,700 were being held as adults.
In 2012, US jails held 5,400 juveniles, of whom 4,600 were being held as adults.

"Note: Data are based on the inmate population confined on the last weekday in June."

Source

Zhen Zeng, PhD. Jail Inmates In 2022 - Statistical Tables. Washington, DC: US Department of Justice Bureau of Justice Statistics. December 2023. NCJ307086.