It sounds like you may have a new account. The limit for how many images you can upload goes up 200 images with every job you run. The initial minimum is 500 images. To run 600 images you would need to have run at least 3 maps already.
This is noted where you select the images:
The goal here is to throttle users with new accounts a bit so they learn the ins and outs of the system. We had too many people signing up and trying to run 4000 image maps without ever bothering to learn what works and what doesn't.
So if you need to run 600 images just make sure you have processed 3 maps in your account already. You could just run a quick 20 image map if you want but we want people to learn to walk before they run.