I think it really comes down to a preference thing. Just as how one may choose to run powered or unpowered, the number of planeswalkers can help give a person's cube its own uniqueness. When you draft Lorwyn, the chances that everyone is going to have the chance to play with multiple planeswalkers is pretty unlikely, so I think trying to recreate that feeling is a reason to limit yourself on how many walkers you include. Even at 5%, you're still way more likely to draft a planeswalker in cube than out of an actual pack draft. On the other hand, you can make it so cube is the only draft format where you do have the chance to play with tons of planeswalkers, which in itself makes it special. I don't include a ton of planeswalkers in my cube, but I've always enjoyed drafting cubes that do, as well.
I'm playing 24 in my 540, so less than the average here at least. There are a few more I guess might be marginal upgrades to cards I am running now but without a strict quota I do try to limit Planeswalker numbers so midrangey value decks don't become too prevalent. Personally I'm always looking to keep aggro relatively strong in my cube, so it seems to temper my urge to just jam in all the Walkers.
After the post-FRF update, I'm running 28 'walkers out of 540, which is over 5%. One thought I had: run no more than one planeswalker of the same name within a color. For example, decide between Ajani Goldmane and Ajani, Caller of the Pride in white (probably Goldmane). However, keep Ajani Vengeant and Ajani, Mentor of Heroes in their respective guilds. I like this better than a rigid one-per-name rule. In the five colors, I have two Ajanis, two Elspeths, two Jaces, and three Garruks. Doing this would bring me down to 23/540 (presuming there's not a saucy new 'Walker in Dragons of Tarkir to add) and give me a few more slots to add other cards.
[180 classic cube]
http://cubetutor.com/cubeblog/993
http://www.cubetutor.com/cubeblog/23690
My Eternal Cube on CubeTutor| |My Reject Rare Cube on CubeTutor| |My Peasant Cube on CubeTutor
I used to write for MTGS, including Cranial Insertion and cube articles. Good on you if you can find those after the upgrade.