How many photons are needed to (obsreve) a body at z=6 ?
Do you mean how many photons must be detected to observe a body at z=6? Or do you mean how many photons are needed to measure the redshift of a body at z=6?
The number of photons needed to observe an object (spectroscopically or otherwise) depends not on its redshift, but on the instrument used, the backgrounds and foregrounds, the object's color, the frequency of observation, etc. You'll need to be more specific.
This would be an example.
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