The councils of the local government units each have quite different ways of being elected, different numbers of councilors and constituencies. I will only list the current system this time, though I already learned that for the PAO the current system was just adopted in 2003.
The Tambon Administrative Organizations (TAO) have a council, in which every Muban covered by the TAO sends two councilmen. Therefore the constituencies are identical with the Muban, and the number of councilors varies widely. The exception are TAO which have only one or two Muban - in these cases there will be 6 councilors, and each Muban has 3 (or 6) representatives.
For the municipalities (thesaban), the number of councilors depends on the municipal level. Subdistrict municipalities (thesaban tambon) as the lowest level have 12 councilors, elected in two constituencies with 6 representatives each. For towns (thesaban mueang), there are three constituencies with 6 councilors each, and for cities (thesaban nakhon) four constituencies. Since 2003, the boundaries of the constituencies are published in the Royal Gazette, see for example the one on the four constituencies of the city Surat Thani from 2007, issued at the upgrade of that municipality.
The councils of the Provincial Administrative Organizations (PAO) differ in size depending on the size of the province. However, unlike the other local government units, in this case each constituency elects one councilor. At least in all cases I checked so far, every district (Amphoe) has at least one constituency. For example, Surat Thani province has 30 constituencies, of which 13 cover a whole district, 3 districts have two constituencies, two district have three constituencies and finally the Mueang district has five.
The special administrative area of Pattaya, an municipality outside the thesaban system, has a council of 24 members with four constituencies, same as a city municipality. The other special administrative area is Bangkok, where both a council for the whole city as well as a council in each of the districts is elected. I'll cover these in more detail later, as we will see he local elections in Bangkok later this year.