In past Super Mario Bros. titles, crowns would replace digits once you collected a certain number of lives. That's true in New Super Mario Bros. 2, as well. The requirements are different, though. You'll see the first crown replace a number once you acquire 1000 lives, a second crown when you reach 1100 lives, and a third one when your tally reaches 1110 lives. When you have three crowns, Mario and Luigi will remove their hats as a special bonus (they'll put them back on if you lose any lives from there, though, until you once again acquire a third crown).