20.
They're even more rare in Pokemon Mystery Dungeon. They're only on some floors in the Joyous Tower, and almost impossible to find. I once played through PMD:RRT on Visual Boy Advance, and I had to use Gameshark codes to survive in the Joyous Tower. But even then, I couldn't find a single Chansey or even Blissey, and had to resort to Jirachi to obtain them.
But still, Chansey is a cute Pokemon, and it's safe now.