You keep 12 slots, even at rank 50.
Rings that give +5 to each stat cost 3 slots each, allowing you to get a maximum of +20 to a stat with these.
Rings that give +7 to each stat cost 4 slots each, allowing you to get a maximum of +21 to a stat with these.
They are not useless. Without the boundary, you would see people running around with +6 attack, +28 strength, +40HP/MP, and whatever earrings give.
It is worth it to upgrade your rings (If you care about that sort of thing). Personally, I think the bonuses it gives is too little for the hassle it is to repair it, so I keep my rings (I have ~20 pieces of jewelry in my retainer) inside my retainer.
4x +5 rings = +12 Defense, +16 Magic Defense, +4 Evasion, +12 Resilience
3x +7 rings = +12 Defense, +18 Magic Defense, +6 Evasion, +9 Resilience.
Edited, Dec 8th 2010 4:36pm by Meowshi