The crown is a placeholder for a 1 in the tens digit place.
From The Mushroom Kingdom,
The WORLD x-x screen shown before starting a level shows Mario's extra life total. If you get more than 9 extra lives, weird stuff starts to happen to Mario's extra life total. For example, if you have anywhere from 10 - 19 extra lives, the tens digit will be a crown instead of a "1." At this point, it's still easy enough to figure out how many extra lives you actually have left, but after [crown]9 lives (19 lives), the numbers turn into letters. While the crown stays where the tens digit would be, the ones digit goes through A - Z, a blank space, a few solid squares, and then the weird symbols start. Interestingly enough, there is a pattern behind this madness. The symbols (as well as the letters and numbers) come from the game's second CHR table, going in order from left to right and top to bottom.
To find out how many lives you actually have after you have the crown, you must first identify the symbol you have next to the crown. Then count (starting at zero - count: "zero, one, two," etc.) your way to that symbol in the table, and then add 10. For example, "[crown]D" equals 23 lives.