Nothing really. The one that resolved last, will be face-up on the field with its effect negated. The end result is that all Traps apart from the original Royal Decree have their effect negated, exactly the same as if there was only one.
However should that first one leave the field, the second's effect will no longer be negated and will now be applied. Because of this, all other traps will still have their effects negated.
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Yes, the effect only works after the effect resolves, so you can activate it in a chain to "Royal Decree". Cards like "Dust Tornado" can even destroy "Royal Decree" as it is chained to "Royal Decree"'s activation. When the chain resolves, "Dust Tornado" will destroy "Royal Decree" and "Royal Decree"'s effect will disappear since it is no longer on the field.
Yes. Royal Decree only negates the resolutions, it does not negate nor prevent activations.
'Negation' can have two different contexts. You can negate a card's activation (meaning the resolution never happens) or just negate the effect (meaning it resolves but does nothing). Solemn Judgment can negate activations, Royal Decree negates resolutions, and also switches off the effects of continuous traps.If you negate the activation or the effect of Normal Traps and Counter Traps, then that means you don't get their effect. If the negation effect destroys them, they will go to the graveyard on destruction, otherwise they will go to the graveyard when the chain finishes resolving.If you negate the activation of a continuous Trap card, then it is unable to remain on the field, and it will go to the graveyard.If you negate the effect of a continuous Trap card, then it will be able to stay on the field, however will do nothing until the negation effect ends. So if it was an active Royal Decree that was negating it, its effects will become active again if Royal Decree leaves the field. This goes for both chaining a negation effect to the activation of the continuous trap, or activating one later on, while the continuous trap is face-up and active.
there is no limit
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.