Should he have one defense against 1.e4 and one defense against 1.d4, or two defenses against 1.e4 and two defenses against 1.d4?
Should he always play the exact same sub-variation of the exact same variation of the exact same defense, or should he learn multiple different variations of that defense? For example, if he plays the Caro-Kann, after 1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4, should he always exclusively play 4...Bf5 (the Classical variation), or should he sometimes also play 4...Nd7 (the Karpov/Steinitz/Smyslov/modern variation) and 4...Nf6 5.Nxf6+ exf6 (the Tartakower/Nimzowitsch/Korchnoi variation) and Nf6 5.Nxf6+ gxf6 (the Bronstein-Larsen variation)?
Too wide and he will be a jack of all trades and master of none. Too narrow and this risks limiting his rate of improvement and he will be too easy to prepare against.
The repertoires of GMs are much wider in the computer era than before, because of the fear of opponents' preparation, which is made much easier by the use of databases.