I don't think there's any upper limit on how many prisoners a holding cell can hold. For the most part, it's meant to accommodate prisoners on a temporary basis while the guards are figuring out what hole to stuff them into. That said, it actually works quite well as a semi-permanent cell for the dozen inmates currently without a personal cell.
As long as their needs are being met, they won't riot any quicker than prisoners with their own cell. The only difference is if they start complaining, they have more of an audience, and their need for defense and [dev]weapon will go up, to protect themselves from the crazy dude who won't stop yelling. So throw some metal detectors along the path from your kitchen to your holding cell, and the worst that will happen is your cell floor will be littered with knives whenever you get new inmates.
Personally, my Holding Cell is 12x12 (including walls). I've thrown four benches, and two toilets into it. For some reason, the prisoners use it as the common room, as I've added some TVs and about a dozen beds to it, to facilitate the semi-permanent status of the inmates. Right after the Sleep activity, there's about an hour of Free Time. Most of my inmates with their own cells make a beeline to the holding cell...and take a nap. So, there's roughly three dozen prisoners in there at that point, and as long as nobody's complaining, there's no negative effect that I've noticed