The attack=cards in your hand multiplyed by 1000. Spells, traps, and effects doesn't work.
Slifer goes in your main deck as is usual for regular effect monsters.
Yes. Slifer the Sky Dragon's ATK is set by a continuous effect that constantly looks at the amount of cards in its owner's hand. The moment the number of cards changes, Slifer's ATK will change.
Slifer's ATK and DEF is X000 where X is the number of cards in its controller's hand. So if you have five cards in hand, Slifer is 5000/5000. The non game-legal print has no effect text so doesn't explain this, but the game-legal version does.
No. Slifer the Sky Dragon's Normal Summon can't be negated, and cards/effects cannot be used in response to its Normal Summon. However apart from that it has no other protection, it cannot negate anything.
Slifer the Sky Dragon does not have an official passcode.
According to the anime, "X000" refers to the number of cards the owner of Slifer the Sky Dragon has. That number replaces the "X" to represent the ATK.In reality, "X000" is not a legal ATK value and will most likely be replaced by a "?" if an Effect Monster version of Slifer the Sky Dragon is ever made. A "?" means that the card's ATK cannot be determined without the use of an effect. Therefore, according to TCG/OCG Rulings, that card (Slifer the Sky Dragon in this case) cannot be searched out in the Deck via cards that depend on ATK of DEF values (such as Sangan or Witch of the Black Forest).
no
There is currently no game-legal version of Slifer the Sky Dragon.
Slifer the Sky Dragon, the original version, a monster with X000 ATK and DEF will be used. Slifer's effect states that it gains 1000 ATK for each card in hand. That means X=the number of cards in your hand. If you have 4 cards, X=4 or 4000 ATK. (The original version of Slifer the Sky Dragon cannot be used in tournament play.)
It is currently uncertain when slifer will be legalized. However, Obelisk The Tormentor and The Winged Dragon of Ra have legalized versions, so the chances of Slifer being legalized as well are good.
Slifer the Sky Dragon is a God and has no code. If you are playing on the PSP get the Yu-Gi-Oh tag force 1 and it will allow you 3 copies of the Slifer card.
Slifer the Sky Dragon does not have an official passcode.