What is the frequency of the highest note on an electric guitar?

do a harmonic on the last fret which by last I mean closest to the neck pickup on the high E string

so if you got a 24 fret guitar your highest note would be a variation of E in standard E tuning. On a 21 or 22 fret guitar it wouldn't sound as great but it would be your highest note.

to do a harmonic lightly press your finger on the string and hit the string with your other hand , some people can do harmonics without moving their fingers off the strings how lightly they press but the second i hit the note i lift my finger off.