PS is a mem monster, it will use as much as you throw at it.
It all comes down to the style of work you will be doing and file sizes
ex: a multi layer project with 50mb+ size files will need more than light retouching.
in any case for non professional work 8gb is becoming the standard.
I do some heavy duty work so I needed 32gb

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*JUST REMEMBER, FOR PHOTOSHOP THE PROCESSOR IS THE MAIN PART USED*