What RAL number is pantone 877 C?

Pantone 877c (coated stock) is a silver metallic ink, which means it can't be reproduced adequately in CMYK. If you don't mind substituting gray for metallic silver, the following formula should give you the closest match:

Cyan............38

Magenta.......27

Yellow..........26

Black.............9

If the ink has been spec'd for a client, make sure the client understands the difference in visual effect in the printed piece.