expression, noun
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.
彼の反対なんて物の数ではない。
His opposition is nothing to worry about.
この程度のミスは物の数にも入らない。
A mistake of this level doesn't even count.
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.