Did many people actually ever play it that way? Honestly, from my experiences with multiplayer games I have trouble seeing something like this work, unless you've got a dedicated group of friends willing to make a commitment to playing and finishing a module together over several sessions (and who's got that?).

Easiest solution: Skip the official campaign. It's not worth your time. Other modules have better stories and more memorable companions.

Next best solution: Play with other human players instead of henchmen. Or adapt to the game and accept that it's more about your own character. More henchmen just means more boring lifestories to read through, more babysitting bad AI, less challenge and less XP per capita. If you play a cleric, druid, arcane magic-user or ranger, you can increase your party size by summoning or taming animals. Or rotate between the henchmen until you've learnt all of their stories and solved all of their quests.

More complicated solution: You could try Lilith: In-game explorer for single-player modules. According to this user it can also be used to recruit all henchmen in the OC. I haven't tried it myself though. I did tinker a bit with the OHS Henchmen System and would not recommend it. It's a nice idea but clunky in execution (e.g. managing your henchmen requires you to allow a little boy into the party who does nothing but call the menu dialogue) and is based on some inconvenient design decisions (henchmen will only join if their level is lower than yours, so won't accompany a lvl 1 character, and they demand money for their services - which your lvl 1 character won't have). Also, if you want to get to know the official companions from the OC, this does not help you much, as it's more abut creating your own henchmen.

I don't know about "many", but some people do. I have done it myself with three friends; not with the OC but with Hordes of the Underdark, the Sands of Fate series and the Eye of the Beholder adaptation for NWN. The general gameplay works fine in multiplayer and is fun; following conversations (walls of text) not so much. But that's a general problem with playing story-heavy games in multiplayer; Divinity OS suffers from it, too. One player clicks through the texts, while the others get bored, because even if the others wanted to, they could not properly follow the conversations due to the lack of MP-friendly UI in conversations. And the long-winded SP monologues don't really take MP into consideration.

I think there is no limit to party size in post-HotU modules; theoretically you could have a party of ten or more, including packmules etc., but they will always be controlled by a bad AI with subpar tactics and pathfinding issues, so more henchmen can actually make the game more frustrating. Some user-made modules include changes to the AI and provide a better tactical experience, but it's never the same as controlling the party members yourself. So yeah, the limitations are hard to overcome.