As of the end of 2007, there were 7,255 subdistrict and 74,944 muban, this means in average each subdistrict contains 10 muban. There are subdistricts which contain no muban at all - if the subdistrict is completely within a city or town (thesaban nakhon and thesaban mueang) the muban loose their meaning and are no longer maintained. But for the other subdistricts the number of muban can vary quite a bit as well.
The highest values I have come across so far are 30 muban in Huai Sak (ห้วยสัก), Mueang district, Chiang Rai, and 29 muban Ban Rai (บ้านไร่), Lat Yao district, Nakhon Sawan. The lowest non-zero number is 2 in Ban Khuan Mut (ตำบลบ้านควนมุด), Chulabhorn district, Nakhon Si Thammarat. However as I still have no complete muban list the above is just a snapshop of my current data, and I also cannot give more statistics yet like the most common number of muban, the standard deviation from the mean value and so on.
Those 30 weren't the highest number: the subdistrict Sap Samo Thot in Phetchabun had 35 muban in 1970, when two subdistricts were carved out of it, as can be read in the corresponding Gazette announcement.