Many of the ratios in this deck have become standard in the Goat Format, particularly the ones in the Spell/Trap lineup.
In standard Goat Format, two is the magic number: it’s the balance between drawing what you need on a consistent basis, but not risking clogging either.
2 Tsukuyomi’s are used, instead of the 1 copy players ran in early 2005. Considering its potential to generate unlimited card advantage with TER and/or flip effects, it’s a card you’ll want to draw in every game and replace if a copy of it dies. However, 3 Tsukuyomi does mean less Premature/Call targets, less field presence and the risk of having a superfluous copy in hand.
Magician of Faith and Magical Merchant are in two’s. It doesn’t run 3 of any flip effect to prevent Nobleman of Crossout from punishing the deck too hard.
Scapegoat at two. Why not 3? Because Airknight Parshath (which counters Goats) became more popular as Goat Format progressed while Jinzo (which Goats counter) became less popular. With 5 LV1 monsters in the deck, Metamorphosis is less reliant on Scapegoat.
Dust Tornado is excellent for (a) protecting your Airknight from Snatch Steal and (b) clearing enemy traps so that your Airknight can +1. But 3 Dusts can result in having too much spell/trap removal while you need something to rpotect your life points.
Book of Moon is excellent for re-using flip effects and hard-countering Thousand-Eyes Restrict, yet in some situations (such as facing against bigger beatdown monsters with higher DEF), it can only avoid an attack (as a temporary -1) rather than outright eliminate the threat.
Sakuretsu Armor can get rid of threats outright and can protect one’s life points without potentially minusing or requiring followup from an attack, but it lacks the versatility and utility of Book of Moon. What if the player faces a stall or flip effect Chaos build that doesn’t often attack; then it’s dead weight.
Between the 4 spells/traps I mentioned above: Dust, Book, Sakuretsu, Goats, all of them are incredibly useful but this deck likes to generally use an even amount of each so the deck can have a response to any situation.
The exception is Metamorphosis, which is used at 3. This is not only because it’s a card that often +1’s the player, but also because the effect of TER is versatile in being able to clear either set monsters or face-up monsters. Using 2 in a Goat Format deck would mean that player who did use a 3rd copy would have an advantage in the late-game.
The two copies of Airknight synergize with the deck on many levels. They make Premature Burial and Call of the Haunted great mid-late game cards. They’re a discard choice for Tribe and fuel for Black Luster Soldier. Even though tribute monsters can clog the hand (which is why 3 aren’t ran), the potential to plus is too good to not run tributes. They convert floating flip effects monsters into a free tribute. They not only give the deck access to Dark Balter via Metamorphosis but also allow the player to tribute their own Thousand-Eyes Restrict (after it had already activated its effect) and make an offensive play rather than just sitting on TER and letting the opponent stall. A player may test the waters with TER just to see if the opponent has Ring/Torrential/Book of Moon/etc. set face-down and then tribute it for Airknight once they have more confidence they can get in an attack with it.