The short answer is that tel ou tel is the singular and tel et tel is the plural. Tel ou tel X and tel et tel X both convey that there are many possible X and it is not known which one(s) will get picked. Tel ou tel X is used when a single X is in play, tel et tel X is used when multiple X will be in play together.

English has a similar construction with this or that vs this and that (and a mostly-synonym phrase such-and-such (so-and-so for people) which can be either singular or plural).