What kind of design are you looking for? Do you want gaps between the LEDs, or one fluid hoop of color? Consider also how the weight difference would be between 20 and 30 LEDs.
My hoop is 35" with 29 LEDs. They light up the entire hoop; no gaps. That being said, it's a PSI hoop with the brightest LED's I've seen around (with the exception being maybe the Atomic). Not all hoop makers use such bright LEDs (although I've heard great things about Astral!)
If you wanted a pattern with minimal gaps, I'd go with 25 LEDs or so - probably not up to 30, unless you really wanna go for smooth coloring. If you like the gaps (some do; they make for nice trails and you can distinguish between colors better) then go for 20 or less LEDs. I know some PSI hoops come with only 16.
Hope this helps. :)
I have a 35" hoop with only 18 LEDs (from Lighted Lifestyles) and I've never thought I needed more. That said, they are all strobing LEDs - it might be more important to have more if you have a solid color pattern going.
I used to have a Trick Concepts hoop that had 32(?) LEDs in it and the thing was so blinding! I hated that I couldn't see anything around me while I was using it at night.