Where do I find model number?

The regulatory numbers have historically been printed on the bottom of the stand.

The traditional human-readable desgination can be found in ->About This Mac.

The "human-readable" version (good one, padam35!) is the most useful because Apple have reused some "model" numbers across years as a form-factor identifier and they may not discreetly describe your computer's internal architecture for parts and service purposes.

Examples:

• iMacs from mid-2007 thorough Mid-2009 all have the same model numbers: A1224 for the 20-inch version and A1225 for the 24-inch versions.
• 13-inch MacBook Pros from Mid-2009 through Mid 2012 are all A1278 and have the same form factor, yet have different RAM needs and macOS limitations.

We don't know why you need the model number but I no longer put much trust in parts/service vendors who only go by the model number. That suggests to me that they may not be as Mac-savvy as they would like me to believe.

Fortunately this duplication has largely been addressed with newer Mac models.