I am sure there will be a limit, but it is probably in the millions. For similar programs, it usually caps out at around 30 million addresses, pubkeys, xpoints...

a.a.
Have you ran this program yet? It's just that some of your answers make it seem like you have not ran it at all.

The program does not check keys in parallel, it runs range with one pubkey, once finished, it moves to the next, until the last pubkey has been checked for that specific range.