According to Official Guinness Records,
The most baskets scored in a game of Rocket League Basketball mode is 99, and was achieved by Jonas Schaffrick (Germany) in Velbert, North Rhine-Westphalia, Germany, on 23 May 2017.Jonas attempted this record in a 1 v 1 match on Rookie mode.
For a complete list of 2017 records, please visit 2017 Guinness Records in Germany.