In MTG, one in every one trillion people end up becoming a Planeswalker. Generally, they are thrust from their home plane immediately upon Ignition; this quest was an exception in that Kyubey still had a grip on your soul and really didn't want to let go. Thanks to what he was doing, he didn't have to.
Planeswalker sparks are awakened by a sufficiently traumatic/life-threatening scenario. Given as there will shortly be an awful lot of those flying around, a vote is necessary.
No matter what option is chosen,
you'd still have to intentionally help the Incubators in order for them to access the other planes. (Unless a write-in suggestion is done otherwise; I wouldn't veto that)
[] Go with the MTG statistics. Allow the existence of 'walkers other than Jade, with one out of every trillion people becoming one.
[] Make the possibility of other 'walkers even smaller, but still there. (1 in 1 quintillion)
(Warning: Above two options may invalidate my earlier promise not to have any hostile planeswalkers. At least they'd be even newer at it than her.)
[] Don't have the possibility of other 'walkers turning up.
[] Suggestion? (Subject to veto)
EDIT: To elaborate, I fully expect for the third option to be picked by a landslide, but wanted to put it out there so people don't complain about statistical inaccuracy later on.
EDIT 2: *Numbers refer to number of people who go through a sufficiently horrible event, not total numbers. It might be somewhat inaccurate (text is a little unclear), but we'd be swamped if the former were a thing.
EDIT 3:
On that note, since a bunch of people are voting for "don't have them", but are fine with them cropping up later: It won't really be relevant for at least several in-game months, possibly years or decades. The other Incubators can't start experimenting until after the complete information from Kyubey reaches them.