We have been told over and over that there are only 2 sith at a time. So are we to assume that "Darth Tyrannus" only came about after Darth Maul was killed? He seems kind of old to become a Sith pupil, but he did kick some but in his time.

He was already a Jedi master, so once he turned to the dark side he wouldn't require much training. Just like Anakin.

Where?

Episode I:

Yoda - "Always 2 there are, no more, no less, a master and an apprentice"
Mace - "But which was destroyed? The master or the apprentice?"

And remember that there was a 10 year gap between Ep I and II, plenty of time to train Dooku.

TPM.
Yoda says about the Sith "Always two there are, no more, no less".

It has been (as related to films):
Darth Plageus / Darth Sidious
Darth Sidious / Darth Maul
Darth Sidious / Darth Tryrannus
Darth Sidious / Darth Vader

I thought the whole "always two there are..." meant always 2 working together, not 2 and only 2 in existence at a given time. like jedi - master and padawan.

That is what I always thought too cause the Sith ruled the galaxy at one point... couldnt have been only 2 dudes running everything.

This probably dips into non-canon sources (since it isn't directly in the films), but...

The deal with the Sith is that there used to be tons of them, but they'd always plot against each other and kill each other and not work together. So for the past 1000, there have always only been 2 at one time, a master/apprentice. As soon as a third gets thrown into the mix, there's death and bad stuff (Palpy having Anakin kill Dooku, Vader trying to convince Luke to overthrow Palpy).

Was that IN the films or just related to?

It's in there.

I remember something about that if there are ever 3 or more sith, 2 of them will nearly immediately overthrow the master. The thirst for power is so huge that one is always trying to kill the other. Even in ROTS, Anakin already has plans to kill Sideous and rule the galaxy with Padme. And when ROTJ rolls around, both Vader and Sideous want Luke to join so he can kill the other.

D

Existing somewhere in the extended Star Wars universe is the story of the second Great Sith War and the reason why there can be only two Sith at any given time. Not two working together, but only two total.

Basically, during the final battle, one of the leading Sith, Darth Bane, (who does appear in the book adaptation of TPM), attacked the Jedi with the combined force powers of all the Sith. It was working. However, the other Sith were frightened by the utter darkness in Bane, so they broke the meditation circle and, if I recall correctly, started to turn on each other. During this lull in the battle the main Sith leader, emboldened by a newly discovered understanding of the power of the Dark Side, used a 'thought bomb' to kill all those present in the battle, including himself, save for one Sith, Darth Bane.

Bane kept the teachings of the Sith alive, but from that point forward, remembering the turn of the Sith against him at that final battle and the constant in-fighting in the time preceeding it, decreed that there could only be two Sith, one master and one apprentice. There would be no repeats.

Dooku turned after Qui-Gon was killed. He originally intended to hunt down and kill Sidious but was seduced into leading the Droid Army. There is some great history in Labyrinth of Evil. The expanded universe has really added alot to the films. Not to mention the Clone Wars cartoon. I wonder how many people will wonder why Grievous is coughing in Sith

Which brings up the next question. Why WAS he coughing?

since grievous was never anywhere else in the movies, people who haven't seen clone wars will just think of it as him being an old battle-hardened general.

In the last episode of the Clone Wars cartoons (which Lucas had them do and gave them details on what should be in it) Mace Windu Force crushes his chest.

So what's the deal with Dooku serving Palpatine while at the same time that black-and-white chick in CLONE WARS, VOL. 1 is serving Dooku? Doesn't this make three?

She called herself a Sith, but Dooku laughed at her and said she was no Sith (if I'm remembering correctly.)

Maybe she was an early attempt at an Emperor's Hand?