Numerousness is the noun form for the adjective numerous.

The abstract noun form of the adjective 'numerous' is numerousness.

The noun form for numerous is numerousness and numerosity. You may want to use the noun many.

Example sentences:

The numerosity of calls has completely tied up our service line.

His generosity is evidenced by the numerousness of his good deeds.

A great number of persons collected together; a numerous collection of persons; a crowd; an assembly., A great number of persons or things, regarded collectively; as, the book will be read by a multitude of people; the multitude of stars; a multitude of cares., The state of being many; numerousness.

Edward Harold Harper has written:

'The effects of selected experiences on the ability of kindergarten and first-grade children to conserve numerousness' -- subject(s): Arithmetic, Mathematical ability, Number concept, Study and teaching (Primary), Testing