Where is the engine number found on a Volvo fm12?

If you're unable to find it, you can call customer service for Volvo Trucks. If you give them the VIN number of your truck, they can look up all specifications and details. Not knowing which country you're in, I'm unable to give you the number specific to your country, and the North American number probably won't be of much use to you, as cabover Volvos haven't been marketed here since the early 90s.