It seems it only accepts numeric values, and although Count() will return a numeric value it's not itself a number. That will work as expected as well with the dollar expansion
SubStringCount(GetFieldSelections([Column Name], ',', $(=Count(Distinct [Column Name]))), 'Item')
That happened in version 10 SR3 update 9061 with the Min() function, when the thrid parameter must be a numeric value instead of an expression returning a numeric value. I use the dollar expansion/macro expansion $(=) there as well.
Hope that makes some sense.