Reading Capital Vol. 1 chs 11 to 13
The supply of exploitable labour doesn't depend just on the no. of workers employed, as even 1 worker can be made to work longer hours
BUT there's only 24h in a day. With 2 workers you have 48 hours to exploit, 10 workers 240 hours, etc
The bigger the operation, the more exploitation can happen
Not just quantitative difference but qualitative: more employees means they work side by side & work becomes factory work