How soon after pups are born do you let those potential owners know that pups have been born ?
Thanks in advance.

usually within 48 horus if all seems well.

With my current puppy, I'm good friends with her breeder and she sent me a text as each one arrived ;-)

I would say that a couple of days after ought to be soon enough. You need to know that mother & pups are well before announcing to buyers that they may be having one. It would be devastating to not only yourself but a buyer should something nasty happen. Unless its someone you know well or are very good friends with, then thats slightly different.

Both my dogs breeders let me know after a day or two

So far mine have gone to breeder friends who have been involved from the start and been at the end of the phone during the whelping to answer questions or checking we were OK.

My next litter I expect to be the same. Some breeders don't let on they have pups till they are at least 4 weeks old so as not to be pressured by folk wanting to visit.

If you plan on keeping one or you have promised one to the stud owner/bitches breeder then you need to make it clear that final picks may not be done till 8 weeks old.