I've used Earthboxes for the past three years. I follow their instructions and put 6 pepper plants per box. The first year they did great, big healthy plants and lots of peppers. The only issue I had was it became top heavy, and blew over, so I had to stabilize them. The next year I bought the organic refill pack, and didn't bury the fertilizer far enough below the surface, and ended up with a container full of maggots in one of the containers!!! In the other container the plants were a bit leggy, but still produced well. Last year, I was trying to save some money and used whatever potting mix I could find locally. Big mistake! There was a ton of leaf drop, and all of the peppers were stunted. I realized the soil was holding way too much moisture, and removed the plastic cover. Once the soil dried out, the plants started to recover, but they were so far behind, that none of them really produced much. I'd like to make my own mix this year.
As far as number of plants per box, I think that depends on what kind of peppers you are putting in there. I certainly wouldn't put one that has a shrub-like habit, like a Lemon Drop, in there, but if you are using it for things like jalapeno type peppers, I think you could fit at least 4 in their without sacrificing productivity.
Just reread the original question. Yes, you could use it for herbs. Probably 6 to 8 basil plants would work nicely. You could also use it for lettuce or other leafy green vegetables. As deep as they are, you could probably grow carrots, radishes, etc. in there too At least you wouldn't have to worry about it being top heavy. About the only thing I wouldn't grow in them again is tomatoes. Tried that a couple of times, but had two issues. We get very strong, sustained winds in the spring here, so they were way too top heavy. Even with them all the way against the side of the house, they still managed to blow over a couple of times a season. The other problem was watering. Two full sized tomato plants would completely drain the reservoir in a day. If I missed even a single day of watering, I ended up with blossom end rot. Of course, I live in a high desert climate, so your results may be better than mine.