That depends on your definition of an AP jungler.
Are we talking about a champ who specifically builds AP, or just a champ who deals significant magic damage?
In the first case I could buy 9 champs, although I think more than 9 are playable 9 viable might be right, maybe a touch low though. In the second case I'd say over 20. In terms of actual use it doesn't matter if I build Devourer and shit tons of AS or if I'm Kayle. Ap bruiser actually play almost the same as tanks, some of them (Amumu) are the same.
The champ's I'd say are legit viable as AP jungles
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Count:12
But there's way more playable champs
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Count:19
That's not to say all these champs are good or should be played with an AP centric build, but they can all clear the jungle with one. Some of the more popular ones like Zyra even fill niches that top AP junglers can't.
If you want to go to deals significant magic damage you can add
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Count:9, although realistically higher because of people like Sej.
Out of all the champs in the first two catagories the only one who can't at all use Runeguard is Fid, Karthus can struggle but he can use it and loves the mana return. It's probably not as good as Magus on Nunu/Malph/Morg/Vlad, but not by much. Every other champ would prefer Runeguard to Magus at least IMO