EXCLUSIVE

XXXTentaction's count of felony charges is now into double digits -- prosecutors added 8 additional, bringing his total to 15, and they're all related to witness tampering ... TMZ has learned.

According to prosecuting documents ... prosecutors believe the alleged witness tampering went down in October 2016 ... when XXX was initially arrested for domestic violence. He's accused of beating his then-pregnant ex-girlfriend.

Law enforcement sources tell us XXX made multiple phone calls while behind bars, which were recorded ... as all prisoner calls are. We're told evidence from those phone calls led to the new charges.

We broke the story ... XXX was sent back to jail last week because the new charges constituted a violation of his bond. Prosecutors believe he tried to coerce the girlfriend into not testifying.