If there are only 5 companions total in the entire game, that would be troublesome, but I don't think that there are that few. This isn't even at the Early Access stage yet, not everything is ready.
Check the banner of the game and listen to Sven saying you're going to meet your companions at the very early beggining of the game. I don't think you'll find other "companions" during the rest of the game exept what they called followers.
We have to hear more about them but I think they"re not going to act like true companions (I bet for followers for specific quests or followers staying in your camp such as a healer, a merchants, a side quests giver, a guideline to the story or temporary important NPC...)