What is the difference between tel ou tel and tel et tel? What is the proper way to convey them in English?
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).