Can anybody suggest how to get Printer serial Number?

sticker underneath has the serial number.

look on the back.

From the manufactures label on the back or base of the printer.

it's on the label -- usually on the back

Unfortunately, sir, without YOU telling us which specific brand and model of printer it's impossible to say as the serial number can be located in just about any position/area of the printer.

Edit: Sadly, it appears that I've upset some imbecile.  Printer serial numbers CAN be found on many areas of a printer ...  some on the outside, either at the rear, or underneath, and some inside the printer itself.  I've been using different brands and models of printer for more than 40 years now so I KNOW what I'm talking about.

all modern printers provide a Web Interface. Just open a browser at the IP address of the printer and you will be presented with its webpage interface.