Is there one? I cant find it in the books?

EG: Can you spend 6 DPs to upgrade your YYG pool to YYYYYG?

Or can you only do that once? from YYG to YYY?

Can the same be said for upgrade difficulty?

Can you Upgrade the Ability AND the Difficulty? (PCs spends one, GM spends one).

Do you have to declare all your upgrades at once, or can the PC and GM upgrade sequentially?

Can PCs spend a DP if they previously didnt have any, and the GM spends one (Flipping it light side), or does it not flip until after the roll?

Limit 1 each on a dice roll. It's under the sub heading "The Limits of luck" FaD p37, AoR p 36 and EotE p28

One per action. P. 27 EoE.

I guess the max destiny points would be players X 2.

As far as using them, I don't think there is a limit. But you have to deal with all the other players who may get irked if you use all their destiny at once.

Edit: I guess I'm wrong on count 2.

It actually specifically states "if you upgrade a check with a DP you can't use another to trigger a Talent on the same action"

So PC's can use 1 per dice roll and GM can use 1 per dice roll. Effectively this means 4 PC's can't all flip 1 each for a single Nemesis roll to upgrade the difficulty 4 times.

As others have said, the rules limit the players to 1 light side point per action and the GM to 1 dark side point per action.

My group added a houserule that whoever is taking the action gets first crack at spending destiny, and if they do spend destiny then the other side must spend 2 destiny points instead of 1 if they want to upgrade that same action. That would obviously be an exception to the above rule but it still allows each side to only upgrade the check/difficulty once per action.

I guess the max destiny points would be players X 2.

Unless there is at least one Light Side Paragon (morality >70) playing. Then the max goes up by one for each.

As long as the character's Morality score remains above 70, at the beginning of every session, when generating the Destiny pool ... the character adds one light side Destiny Point to the pool.

* - One light side DP for each LSP, plus each character getting either 1 or 2 pips of light or dark.

** - Of course, each Dark Side Paragon (DSP) then flips a light to a dark after the pool is made, so no real guarantees!

I would also say you can spend one on a roll and then one on flipping force pips to light side.

I would also say you can spend one on a roll and then one on flipping force pips to light side.

While that seems pretty reasonable, I'm pretty sure it's not RAW.

Keep in mind that Signature Abilities initially require the use of TWO destiny points to activate. AFAIK that's the only situation where a player is allowed to flip more than one by RAW.