If you can find no App Store subscriptions connected to any of your devices, then it's most likely a scam.
https://toughnickel.com/scams-fraud/Apple-iTunes-866-xxx-xxxx-Scam
https://finance.yahoo.com/news/check-credit-card-statements-scam-172745615.html
https://candid.technology/866-xxx-xxxx-scam/
https://wntoknow.com/why-do-you-see-866-xxx-xxxx-on-credit-card-bill/
Does the charge line on your credit card statement match any of the following used by scammers?
These are all of the known scam charge lines. There may be more.
CHKCARDAPPLE COM BILL CUPERTINO CAUS
CHECKCARD APPLE COM BILL CUPERTINO CAUS
POS Debit APPLE COM BILL CUPERTINO CAUS
POS PUR APPLE COM BILL CUPERTINO CAUS
POS PURCH APPLE COM BILL CUPERTINO CAUS
POS PURCHASE APPLE COM BILL CUPERTINO CAUS
POS REFUND APPLE COM BILL CUPERTINO CAUS
PRE-AUTH APPLE COM BILL CUPERTINO CAUS
PENDING APPLE COM BILL CUPERTINO CAUS
Visa Check Card APPLE COM BILL CUPERTINO CAUS MC
Misc. Debit APPLE COM BILL CUPERTINO CAUS
CHKCARD APPLE COM BILL CUPERTINO CAUS
APPLE.COM/BILL 866-712-xxxx CAUS
APPLE.COM/BILL 866-712-xxxx, CA, USA
APPLE COM BILL CUPERTINO CA
APPLE COM BILL CUPERTINO CAUS
APPLE.COM/BILL INTERNET CHARGE CA QUAN B NGUYEN -88006
APPLE.COM/BILL IRL CORK
APPLE.COM/BILL wwx.aplle.cxx CA
(had to x out some of the last one to prevent it from being an active link)
If you have charges on your credit card for this charge line, but no matching items actually charged by Apple, then your credit card has been compromised. Contact your card issuer. Have them reverse the charges if possible, then immediately cancel that card and have them issue you a new one.
Sorry, the links above do exist, but they keep changing the URL. I can't fix them right now.