Not less than five, perhaps no more than 10-15, more than enough to cause trouble, particularly if materials are given to another of our enemies. DPRK’s attitude at this time is, enemy of my enemy is my friend. They could even consider outreach to Hamas, Hezbollah or even al-Qaeda if necessary, and desperate for cash.
Since the two that they have detonated (Oct 9, 2006 and this one) are different, my guess is that they do not have ready made bombs in storage, they are still under development. Thus they probably have plutonium for less than 10 devices, but they have to assemble them as well.
http://www.carnegieendowment.org/publications/index.cfm?fa=view&id=16912