At two places. You should have gotten a Plastic-Card with your frame number printed on and also your dealer should have mentioned it on the receipe/incvoide.
And then there is a little QR Code Sticker you should find on the inner side between battery and your bike with the number on it (you should see it when you remove your battery)
There is a FAQ by BMC:
Where can I find the BMC serial number? / FAQs
Where can I find the BMC serial number?www.bmc-switzerland.com