There is no limit to the amount of text that you can fit on the screen. It will just keep getting smaller and smaller and smaller as you add more and more and more text to the slide. If there is any more room left over than you can keep adding more text and it will continue to get smaller and smaller. Obviously you will want to cut it off at some point because you don't want a whole bunch of text taking up the entire slide it will not look good any more.