If you want to know what the number is of the specific MIDI note on your instrument you want to play with the Arduino, you need to look up the Implementation Chart of your particular MIDI device online.
When writing code for the Arduino for a MIDI interface, you need to specify the instrument key, which is a number hardcoded in your electronic musical instrument. Our interface products use Arduino as the microprocessor for controlling things, and, lucky for us, there are MIDI libraries for the Arduino.