How many top tier do we need ?

A sentence in the safe bets for gen 3 article keep on getting me thinking so I had to come here and ask. I quote : "before you work on maxing out your next Tyranitar, for example, take a hard look at your roster and ask yourself what another maxed out Tyranitar will do for you currently in the game"
How many powered up t-tars is enough ? How many of each top tier do we need ? How many d-nites, machamps, golems, vaporeons... If we are thinking long term here. Basically at what point would you not power up a hatched 96 or 98 or heck even 100 larvitar because you already feel like you have enough powered up t-tars ? I always figured you should not stop until you have 6 because it's the max number of slots for raids and gym battle and if that particular mon is the supreme counter ideally you want to use only this one. But 6 is maybe too extreme idk.

Also this quote make me want to ask what are you supposed with your stardust ? It's only value is when it is spent on your mons. If you are not going to use it to power up t-tars what are you going to do with it ? I can understand saving for future generations even tho I think we all realise how slow niantic will release them all. I can understand saving until you get stronger iv's. I can understand not powering up a strong iv useful mon because you already have enough of them powered up. But having the goal of accumulating as much as possible I don't get it. Yet I see people, i see youtubers who sound like the main goal of the game is to build the largest stardust stack possible. I think of prodigiesnation or poké AK or that less famous youtuber who has 12 million stardust.