What we need is to set limits on how many embryos can be formed in vitro, i.e. no more than will be implanted, no "spare" embryos. That, and encouraging embryo adoption would eliminate the problem of discarding "leftover" embryos.
I think the problem with that is that you don't know how many embryos will be created during the in vitro process. But I'm quickly getting out of area of expertise here and will defer to others.