It's a breach of contract, plain and simple. If he had a written agreement that his sperm would be used to father 5 children and no more, and the clinic violated that, then he is entirely within his rights to sue the clinic for reneging on its half of the deal.

As for guaranteeing a specific number, first, if they couldn't guarantee that, why would they agree to the limit in the first place? And even if the guarantee was violated without intent, I would expect perhaps 6 or 7 children... not 17.