How many ttars are enough?

Six maxed each.

More seriously, depends on your local meta and goals. I have two with Bi/Cr and one with SD/SE at level 30. Holding back on further investments due to impending Gen IV release.

For me personally, three each.

The other three slots can be augmented with other mons such as Houndoom for Dark and Golem for Rock.

I really wish I hadn't missed larvitar community day...

I have 6 of each at level 30 or higher, but that's probably excessive. 3-4 is probably fine.

You can't answer that question until gen 4 is out.
For rock type Ttar, we need to see the moves Rhyperiors gonna get.
For dark type, you might want to leave 2-3 slots for darkvire.
I did not power more then 4 (3 dark types) for that reason.

I have 6 each
All level 35-40
But i try to duo Uno,Dos,Tres and trio Mewtwo so i have to..

Depends entirely on what you need to do with them. I'd say 3-4 level 30 Tyranitars for both sets is completely fine and you'll rarely need more than that.
For shortman raids, you need 6 very high level rock Tyranitars. For Alakazam solos, at most you need 3 level 35+ Tyranitars. That's about it for the things where it truly matters.

I had 3 dark before comm-day, now 6 rock and 4 dark.
one of each maxed out. two at 35 and rest is between 30-35 (simply weather boosted strong or shiny ones).
When something new or better comes out I can easily replace wb ones and keep using my best ones along with other counter options.

In the end its both 1 of each, 3 of each and 6 of each at the same time

I'd say 2 dark, 2 rock is more than enough. If you were lucky to play its community day and have two SD. There is no current use for a TTar army.

No current raids require dark TTar (unless you want to trio Mewtwo) and rock TTar can still be replaced with rock golem. In the near future there will be added several other rock types, potentially as good as TTar. I would save my stardust for some of them.

I have 1 rock and 1 dark maxed and 5 of each level 30. But 3 is the most i used in a team and 2 of each is probably enough.

My son had an interesting perspective about this: it may be better if you run fewer (3 was the suggested number) in any given battle if you have a potion/revive drought. That way you can make sure you can keep them revived and healed over other options which can be healed as able.

I would say 5 dark max (darkrai) and I would wait until gen 4 before maxing any rocks,(very little exclusive use in current meta)