Rock Tyranitar: Quantity vs ... Power?

I'd max out your best ones one at a time before working on the lower ones, dps is the most important thing for gyms and raids, and higher levels means more dps

However, with gen IV coming, you may want to invest more in Bite/Crunch sets, as theres a grand total of 5 legendary pokemon weak to dark, not to mention the already existent mewtwo EX raids, but 1 weak to rock (and that one is double weak to ice, and can super effect tyranitar, its sky shaymin, who may or may not even be added for a long time)

You may however want tohave enough saved up to max at least one SD in case rayquaza comes back if you dont have any sufficient ice or dragon types if the weather isnt partly cloudy often/wont be able to small group it

I guess it depends on your other rock pokemon. Like you said, rarely will you need a team of 6 rock attackers. I normally max out one of each type of pokemon. The exception to that rule is if I get a 100% after I already maxed one, so for example I have 2 maxed Machamps and 2 dark Ttars. I would personally rather invest in a Golem than another rock Tyranitar. Alolan Golem golem double resists flying which makes in an interesting option.

Typically, I will always suggest 1 maxed over quantity. However, Ttar is one of my few exceptions:

GIven that SD/SE at level 28 is better than ANY OTHER rock set currently available, I would recommend as many at or above that level as you can manage. "Usually don't need a full team, raids don't often have enough trainers," is not a good enough reason not to develop your teams. It's not about damage balls necessarily; it's about being able to hold your own and do the tough raids well.

NB I value doing damage over worrying about the revives you might need if you run 1-2 full teams; I tossed 275 revives earlier today because I needed bag space and we get so many Max revives from gifts now. I would also rather be able to do the borderline raids (4 people for Registeel versus the 6+ that the local meta feels is necessary; one group I'm part of was even discussing 3+ as a minimum for the soloable Raichu...), not for prestige sake, but for the challenge.

Yes if you always have enough people you can go with your optiom.
Even better, you dont have to do anything with that Rock TTar, you already set..

For trainers that cannot always play with a group, it can be a different story
Or some trainers that actually love battle solo or duo..
Like uno, dos, tres, 3 level 35 Rock Ttar for duo raid is not enough..

One of the articles on this site actually had a pretty good rule of thumb regarding maxing out vs having an army at a lower level;

In the short run, if you have urgent raid needs, it’s better to have more. A team of 3x L30 Articuno and 3x Cloyster performs better than a team of 1x L40 Articuno and 5x Cloyster.

In the long run, or if you don’t have urgent raid needs, it’s better to have few. This way, when the better alternative arrives in the future, the maxed veteran will still have some place in the 2nd-6th slot. Had they been at level 30, they would be outclassed immediately and you would have a bigger to-transfer-or-not-to-transfer issue. A caught Kyogre is better than a level 30 Gyarados but not a level 40 one. An army of L30 Gyarados is collecting dust, while L40 Gyarados may still have a place in your Water squad.

With Zapdos's passing there isn't an immediate need for rock types right now. I'd rather max one and be done with it rather than build up an army, especially if you have other more urgent stardust needs.

I'd wait until gen4 until powering up ttar, as both rampardos and rhyperior could outclass it depending on their moveset. so unless you have an imidiate use for it hold back your resources