They do tend to run out of things as the flight goes on, so if there's anything you particularly want, ask for it early. I once asked if I could have a dish that looked popular after I'd had a quick sleep, and I'm quite sure it would have run out had I not done so.

As for eating multiple quantities of any given item, no problem whatsoever. After ordering a second starter that I liked, I was pro-actively offered a third.