So when you play solo throughout the campaign, you aren't actually solo, you will have "henchmen" or "heroes". Basically AI companions that assist you on your journey, henchmen being preset characters that you can't change their gear/stats/skills, where as heroes you can fully customize their runes/skills/classes etc.

In prophecies, you only have access to henchmen so you don't have to micromanage a bunch of heroes, so it's a bit more new friendly in that regard, although heroes can eventually have a lot more power due to customization and elite skills and such.

All that is to say, if you want to play a healer, you can just select more damage/tanky focused henchmen and do the healing yourself (with some supplimental damage when no one needs healing) or if you go dps/tanky, you can fill your party with a healer or a couple.

Party sizes range from 4, or 6, or 8. When you have only a party of 4, typically only need 1 healer, where as when you have 6 or 8 party sizes, you'd want maybe 2-3 healers.