I currently have 10 and am always struggling to make space. I do sell a lot on the market and try to keep all crafting mats on hand as well. There are just so many things that I have on them. I struggle to keep a lot of different things because I hate having to go farm it again. I don't see why they don't let us have more retainers...... Or better yet, a crafting chest for all mats to go into. That would free up probably 4 or more retainers there.
If they were to add more retainers on top of the 10, it's just more money in their pockets imo. I would gladly pay it.

Even if the crafting box is linked to a players house. But then you have to actually win the lottery to get a house first nowadays, thankfully I have mine from forever ago. Just need something to add more space.