depends on your memory of your machine, assuming your using 64bit. Then I would say maybe a billion or 2 billion, yes BILLION should be fine.
I definitely work with hundreds of millions at times.
Scrapebox V2 is 64bit so as long as your using the 64bit and your machine is 64bit then the limit is the amount of memory you have in your machine. But yes Scrapebox can handle 10 million no problem, unless your running really low memory then maybe not.
Plus even if you run out of memory, the results are saved in the harvester session folder in real time so you don't actually lose them.