I was curious so here are the playable characters in Jump crossover games over the years. Ignoring support characters:
Jump Ultimate Stars (56 Battle Characters): Eve, Train Heartnet (Black Cat), Ichigo Kurosaki, Rukia Kuchiki, Tōshirō Hitsugaya, Renji Abarai (Bleach), Bobobo, Don Patch (Bobobo), Kazuki Muto (Buso Renkin), Allen Walker, Lenalee Lee (D.Gray-man), Arale Norimaki, Dr. Mashirito (Dr. Slump), Son Goku, Vegeta, Son Gohan, Gotenks, Piccolo, Freeza, Majin Buu (Dragon Ball), Kenshiro, Raoh (Fist of the North Star), Gintoki Sakata, Kagura (Gintama), Taikoubou (Hoshin Engi), Gon Freecss, Killua Zoldyck (Hunter x Hunter), Jotaro Kujo, Dio Brando (JoJo), Tsuna Sawada (Hitman Reborn), Kinnikuman (Kinnikuman), Kankichi Ryotsu (Kochikame), Neuro Nogami (Neuro: Supernatural Detective), Toru Muhyo (Muhyo & Roji), Naruto Uzumaki, Sasuke Uchiha, Sakura Haruno, Kakashi Hatake (Naruto), Fuusuke (Ninku), Monkey D. Luffy, Roronoa Zoro, Nami, Sanji, Nico Robin and Franky (One Piece), Jaguar Junichi (Pyu to Fuku! Jaguar), Himura Kenshin (Rurouini Kenshin), Pegasus Seiya (Saint Seiya), Momotaro Tsurugi, Heihachi Edajima (Otokojuku), Yoh Asakura, Anna Kyoyama (Shaman King), Yami Yugi (Yu-Gi-Oh), Yusuke Urameshi, Kurama and Hiei (Yu Yu Hakusho).
J-Stars Victory VS(39 Playable Characters): Koro-Sensei (Assassination Classroom), Tatsumi Oga (Beelzebub), Ichigo Kurosaki, Sosuke Aizen (Bleach), Bobobo, (Bobobo), Taro Yamada(Chinyūki: Tarō to Yukai na Nakama-tachi ), Arale Norimaki, Dr. Mashirito (Dr. Slump), Son Goku, Vegeta, Freeza (Dragon Ball), Kenshiro, Raoh (Fist of the North Star), Gintoki Sakata (Gintama), Meisuke Nueno (Hell Teacher Nube), Gon Freecss, Killua Zoldyck (Hunter x Hunter), Jonathan Joestar, Joseph Joestar (JoJo), Tsuna Sawada (Hitman Reborn), Kankichi Ryotsu (Kochikame), Medaka Kurokami (Medaka Box), Naruto Uzumaki, Sasuke Uchiha, Madara Uchiha (Naruto), Monkey D. Luffy, Portgas D. Ace, Boa Hancock, Akainu (One Piece), Himura Kenshin, Shishio Makoto (Rurouini Kenshin), Pegasus Seiya (Saint Seiya), Momotaro Tsurugi, (Otokojuku), Kusuo Saiki (Saiki K), Luckyman (Tottemo! Luckyman ) Toriko, Zebra (Toriko), Yusuke Urameshi, Hiei and Younger Toguro (Yu Yu Hakusho).
Jump Force (40 Playable Characters Base, 49 with DLC): Asta(Black Clover), Ichigo Kurosaki, Rukia Kuchiki, Renji Abarai, Sosuke Aizen, Tōshirō Hitsugaya, Grimmjow Jaegerjaquez (Bleach), Ryo Saeba (CIty Hunter), Son Goku, Vegeta, Son Gohan, Trunks, Piccolo, Freeza, Cell, Majin Buu (Dragon Ball), Dai (Dragon Quest), Kenshiro, (Fist of the North Star), Gon Freecss, Killua Zoldyck, Kurapika, Hisoka Morow, Biscuit Krueger (Hunter x Hunter), Jotaro Kujo, Dio Brando (JoJo), Izuku Midoriya, All Might, Katsuki Bakugo (My Hero Academia), Naruto Uzumaki, Sasuke Uchiha, Boruto Uzumaki, Kakashi Hatake, Gaara, Kaguya Otsutsuki, Madara Uchiha (Naruto), Monkey D. Luffy, Roronoa Zoro, Sanji, Sabo, Boa Hancock, Blackbeard, Trafalgar Law (One Piece), Himura Kenshin, Shishio Makoto (Rurouini Kenshin), Pegasus Seiya, Dragon Shiryu (Saint Seiya), Yami Yugi, Seto Kaiba (Yu-Gi-Oh), Yusuke Urameshi and Younger Toguro (Yu Yu Hakusho).
Important to note that Ultimate Stars wasn't an a 3D arena fighter like the other 2. It was a Smash Bros-style 2D fighting game that worked with sprites. So it was probably easier to include more characters in that.