You can amend as many times as necessary. But unless you enjoy the process (few people do), you'll probably want to hold off on filing any more amendments until you're reasonably sure that you've received all your forms.
As taxpayers, we get surprised sometimes by receiving forms we didn't know we'd get. But if this year is somehow special for you, and this keeps happening, I'd advise you to wait a little while in order to make your next amendment your last.
