To reiterate what Circus Peanut said, this question was answered quite thoroughly in Measure of a Man. It's not that Starfleet can't build more Datas, necessarily. It's a matter of the moral questions involved making it untenable to try to build an entire race of androids.
the moral question was about them being slaves. What if they built thousands and thousands of Datas and let them make their own choice about whether to join Starfleet or not? That solves the slavery issue by having them volunteer.