In ColladaMax, we use the map channel index as the set number. In ColladaMaya, we have names for the UV sets, so we index those in an ordered fashion.
It is my understanding that set numbers are just used to group together TEXCOORD, TEXTANGENT, TEXBINORMAL and other sources. They have no implied ordering, so that it is legal to have two TEXCOORD inputs with set=“5” and set=“182”.
You should therefore support any value: as long as it is greater or equal to zero.