At least, I've always heard that there are ten of them -- but I couldn't list all ten if you held a gun to my head! Let's see, there's the words for "time," "name," "tribe," "burden," "flame," "banner," erm... "udder"... "stirrup," um...Then you've got singular neuter nouns ending in -мя, like время, "time." There's an argument for calling such nouns a separate declension -- but then again, there are only TEN such nouns in the entire modern language
Can't remember what the other two are, but supposedly, there's ten.