“Adonijah, the son of Haggith.” This formula, “person, the son of someone,” usually designates the son and who is their father, but David was Adonijah’s father; Haggith was his mother. David had a number of wives, and their children became rivals.
“chariots.” This could also be translated “a chariot” but it is likely a collective singular.
“he prepared chariots and horsemen for himself, and 50 men.” Similar to what his half-brother Absalom had done (2 Sam. 15:1).