The octave number is in the left column. To find the frequency of middle C, which is C4, look down the "C" column until you get to the "4" row. Middle C is 261.6 Hz.
Some Specific Notes
Middle C: C4=261.6Hz
Standard tuning fork A: A4=440Hz
Piano range: A0=27.50Hz to C8=4186Hz
Guitar strings: E2=82.41Hz, A2=110Hz, D3=146.8Hz, G3=196Hz, B3=246.9Hz, E4=329.6Hz
Bass strings: (5th string) B0=30.87Hz, (4th string) E1=41.20Hz, A1=55Hz, D2=73.42Hz, G2=98Hz
Mandolin & violin strings: G3=196Hz, D4=293.7Hz, A4=440Hz, E5=659.3Hz
Viola & tenor banjo strings: C3=130.8Hz, G3=196Hz, D4=293.7Hz, A4=440Hz
Cello strings: C2=65.41Hz, G2=98Hz, D3=146.8Hz, A3=220Hz
Keep in mind that everything here is in relation to the even-tempered or equally-tempered scale, where an octave is a frequency ratio of exactly two and a semitone is a frequency ratio of exactly the twelfth root of two. In the real world, however, many different temperaments may be used - see en.wikipedia.org/wiki/Musical_temperament - and octaves too can vary in size, see en.wikipedia.org/wiki/Stretched_octave.
I call middle C "C4." This is the most octave numbering but some people call middle C "C3" or even "C5."