Let's make this list a little bigger. I started bosshunting and already found some bosses with more spawns, tbh almost all Nemesis bosses brings "reinforcements" when they spawn:
All Vampire Lords appears with Vampires and can also summon Vampires.
Battlemaster Zunzu -> Appears with 2+ Lizard Zaogun.
Big Boss Trolliver -> Summons 2+ Trolls.
Captain Jones -> Appears with 2 Pirate Skeletons.
Fleabringer - Appears with 2-4 Insect Swarms.
General Murius -> Spawns with several Minotaur Guards and Archers (don't remember if he also appears with Mages, but Guards and Archers I'm certain).
Gravelord Oshuran -> Summons 2 Bonebeasts.
Hairman the Huge -> Appears with 2 Kongras.
Rukor Zad -> Appears with 3 Assassins.
Smuggler Baron Silvertoe -> Appears with 2 Wild Warriors.
The Evil Eye -> Spawn with several Elder Bonelords (5?) and Bonelords (2?), can also summon lots of creatures (even Elder Bonelords).
The Welter -> Summons Eggs (up to 7) that if don't destroyed, turns into Spawn of the Welter, which heals the boss frequently. The Eggs and the Spawns need to be killed first so you can kill the boss.
Warlord Ruzad -> Appears with 3 Orc Leaders and can summon 2-4 Orc Berserkers.
Yaga the Crone -> Appears with 1 Witch.
Xenia -> Appears with 2 Valkyries.
There are other bosses like White Pale that I don't remember if it appears with creatures or if it summons, but probably can summon too like Rotworm Queen.
If I meet more bosses I will come to update my comment.
Edit 1: I've met the Frog Prince! He spawns with more than 3 Green Frogs, I could count 4 to 6 before they ran away, it's certainly more than 2 or 3.
Edit 2: Rukor Zad appears with 3 Assassins, not 4.