Probably need to better define it. "success" is really an individual determination. You've also got to define "group."
Try it this way, how many would make you happy? How big a "group." Not being facetious, just defining the question.
How many people have to buy the Book 1 before you would feel comfortable calling it a success? 5, 50, 100, 1000, 5000?
ETA: Back in 2007
Tess Gerritson pointed out that 5,000 copies in a week would get you on the NYT Best Sellers list.
Or would it be better to ask how many bought book 1, and how many of those folks bought book 2? (See what that does to the question?)
Now, let's go with the 1000 mark. Even if NONE of them were paying, but all of those same 1000 folks who read book 1 came back and read book 2? And let's say they told enough folks that the number was up to 1250. Do you feel that, as a writer, you are a success? Why? Why not?
Like I said up top, it's an individual determination. If only because the only yard stick worth measuring yourself against is the one in your head.
Good luck.
ETA: Welcome to the 'Cooler by the way. I hope to see some of your work in SYW some time.
Also, this thread may get moved to AW Roundtable. That would be the individual room mods call. Don't sweat it if it does.