From the Super Mario Wiki
There are at least five Dragon Coins littered around every level in the game (except the Chocolate Secret level, which only has one, not including the secret area accessible as Yoshi). The first four coins collected in the game results in 1000, 2000, 4000, and 8000 points, respectively. Collecting five of them is the equivalent of collecting 100 Gold Coins, awarding an extra life for the player. On rare occasions, one may encounter six or more Dragon Coins in a single level; in these cases, every Dragon Coin after and including the fifth will award an extra life. In Super Mario World's Game Boy Advance remake, Super Mario World: Super Mario Advance 2, the amount of Dragon Coins acquired are recorded on a menu, and if the player collects them all, the Coins will transform into Peach Coins.
Most of the games in the Yoshi series (such as Super Mario World 2: Yoshi's Island and Yoshi's Island DS) include Coins (both in-game and in official artwork) which have Yoshi heads shown on them rather than having the normal Mario coin design. This makes them look exactly like smaller Dragon Coins, and may suggest that all Coins found on Yoshi's Island were at least once Dragon Coins.
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