To make it clear why you can use only one Telepass (Gardevoir's power) but multiple Download (Porygon2's power) per turn...
Gardevoir reads "Telepass - Once during your turn (before your attack), you may search your opponent's discard pile for a Supporter card and use the effect of that card as the effect of this power. (The Supporter card remains in your opponent's discard pile.) You can't use more than 1 Telepass Poke-Power each turn. The power can't be used in Gardevoir is affected by a Special Condition. (Poke-POWER)"
Porygon2 reads "Download - Once during your turn (before your attack), you may discard a Supporter card from your hand and use the effect of that card as the effect of this power. This power can't be used if Porygon2 is affected by a Special Condition. (Poke-POWER)"
Once during your turn makes it so each card can use the power once (So each Porygon2 can activate Download, but only once per card. 2 Porygons on bench means you can use 2 Downloads. If one of the Porygons isn't able to activate Download via Imprison marker, then you can only use 1 download, etc.)
Gardevoir specifically lists the pokepower as being able to be used once per turn, meaning no matter how many Gardevoirs you have, you can only use one Telepass (rather then once per card). Look for the "You can't use more then one *this pokepower* per turn" text to tell the difference.
Finally, to answer the body...
Darkrai LV.X is a good example, which lists "Dark Shadow - Each basic D Energy card attached to your D Pokemon now has the effect 'If the Pokemon Darkness Energy is attached to attacks, the attack does 10 more damage to the Active Pokemon (before applying Weakness and Resistance).' You can't use more than 1 Dark Shadow Poke-Body each turn. (Poke-BODY)"
Basically, that says that having two Darkrai LV.X out won't double the effect. It doesn't matter how many Darkrais are out, you only increase damage by 10 per energy card.