Where can I find the Serial Number?

Serial numbers are printed on the original product box, usually on the narrow side. The serial number will start will usually start with "E0000..." 

If you don't have the original box, you can find it on the robot itself:

N79, N79S, N79SE or N79W: slide out the dustbin and look inside the robot for a silver sticker. 

DEEBOT 900/901/907: look on the bottom of the robot.

DEEBOT 600/601: slide out the dustbin and look for a white sticker.

OZMO 930/937: remove the dustbin under the top cover, and look inside the robot.

DEEBOT Slim/Slim2/SlimNeo:  look on the bottom of the robot.

DEEBOT R95/R98: remove the dustbin under the top cover, and look inside the robot.

M80Pro, M81Pro, N78: remove the dustbin under the top cover, and look inside the robot.

All other robots:  It's either on the bottom or remove the dustbin and look inside the robot.