You can't do anything, that I know of, to get them back. Don't stress about it. There's plenty of work to go around. As for your second question, as many as you can connect. There may be a limit to how many unique IDs the WCG servers can handle per account, but it's going to be such a ridiculously high number that nobody will ever reach it. I'm talking in the BILLIONS btw, possibly more. If you think you can get a hold of a few thousand machines legitimately, go for it.
If you write a virus and use a whole botnet to crunch WCG, expect to be banned from the project and every forum associated with it for life. Yes it's that serious.
That's not an accusation, that's a warning for ANYONE thinking about it. Don't do it. Just don't.