I think the number of fish can vary on success...with water changes, sumps, refugiums, skimmer choice etc....
I do see a lot of algae so there is obviously some nutrient issues with this tank so I do wonder if 12 is too many....I think he is pushing his limit...just IMO....
I however have what would be considered too many in my 65g but am diligent with water quality and my skimmer/sump are excellent....