something worth mentioning; something to count on; something problematic
Almost always used in negative constructions like 物の数ではない or 物の数にも入らない, meaning something is not worth considering, not a real factor, or not a problem.
His opposition is nothing to worry about.
A mistake of this level doesn't even count.
Standard kanji spelling for this expression.
Kana spelling is common in everyday writing, especially when the expression is used in its negative form.
Literally 'the number of things'. The phrase 物の数ではない (not the number of things) evolved to mean something is so insignificant it doesn't even count as a thing.