Torque required to rotate a set of gears

Hi everyone.
How do i know the ammount of torque needed to rotate the first gear in the grid?
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The gears are not attached to a motor, and the aspect ratio between the big/small gears are 1/5 ( ach complete rotation of a big gear rotates the small one 5 times).
How do i know how many sets of (small/big gears) its possible to manually rotate the first gear ? (I am ommiting the number of teeth, module and etc, because i don't have the data)
I looking for the equations, not the result of the calculus,clues to look for this answer...
(PS.: English is not my native language, sorry any mistakes with terms...)