The basic principles are: don't choose a number of rounds; instead, choose the amount of time password verification will take on your server, then calculate the number of rounds based upon that. You want verification to take as long as you can stand.

For some examples of concrete numbers. A reasonable goal would be for password verification/hashing to take 241 milliseconds per password. That still lets your server verify 4 passwords per second (more if you can do it in parallel). If this is your goal, about 20,000 rounds is in the right ballpark.

However, the optimal number of rounds will change with your processor. Ideally, you would benchmark how long it takes on your processor and choose the number accordingly. This doesn't take that long; so for best results, just whip up the script and work out how many rounds are needed to ensure that password hashing takes about 240 milliseconds on your server (or longer, if you can bear it).