Question - How many cards should be in my deck and why?

The Answer to this question is quite simple

Most "Good" qualified decks rely on 40 cards, why? because whenever you draw a card, you want a better chance of drawing one that will help the situation your in.



For Example the following cards are on your field:

Skilled Dark Magician (1900/1700) [For every time you or your opponent activates a Spell Card, this Card gains 1 Spell Counter (Max. 3) You can tribute this Card with 3 Spell Counters to Special Summon 1 "Dark Magician" from your Hand, Deck or Graveyard]

You have a Skilled Dark Magician on the field and he has has 2 Spell Counters on him out of 3. You have a Dark Magician in your Graveyard your opponent has a Face-down on his/her Spell/Trap Zone as well as a Red-Eyes B. Dragon and does not attack you this turn because you Activated Swords of Revealing Light Last turn (Your Opponent cannot Attack for 3 turns after the Activation of Swords of Revealing Light

So you want to be able to draw the following:

1 ) Monster with a Higher attack then Red-Eyes B. Dragon

2 ) A Spell Card to Activate Skilled Dark Magician's Ability to Special Summon your Dark Magician from your Graveyard.

So if you can make sure you won't draw a useless card by reducing the deck to 40 Cards and having a more likely chance of summoning the right Monster and defeating your Opponent.

A good card to draw in this situation would be a Mystical Space Typhoon, to destroy the face-down Spell/Trap Card and gaining your 3rd Spell Counter, Allowing you to Special Summon Dark Magician without having to worry about the Face-down card.