My friends and I play casual Magic, and I just have a couple questions.
- How many land cards should be in an average deck? I'm betting on ~20 but I'm not sure.
- How does the Soulbond ability work exactly?
It depends on if your deck's mana cost curve and how much card drawing/filtering you have in your deck. In a typical Standard-legal aggressive deck (meaning your curve isn't stupid low like a Legacy aggro deck would be) with no card filtering or drawing, you probably want somewhere between 20 and 22 lands. As your deck's mana curve and card advantage options increase, you want to add more and more mana sources (not necessarily lands - always keep in mind keyrunes and other mana sources that are available) so that you can continue to hit your land drops up to turn 6 and beyond. Whereas an aggressive deck will usually top out its curve with only a few 5 drops (thundermaw hellkite comes to mind), a midrange deck may have several 6 or 7 costing spells or may want to cast multiple spells in a turn. When you're playing a slow control deck with massive sources of card advantage like Sphinx's Revelation, it is not unheard of to play 27 or more lands (in alara-zendikar block standard there were decks that performed very well with over 30 mana sources, because cards like Noble Heirarch and Celestial Colonnade performed dual service as mana sources and threats).
As for Soulbond, this is the entire section of the comprehensive rules on Soulbond:
702.93a. Soulbond is a keyword that represents two triggered abilities. "Soulbond" means "When this creature enters the battlefield, if you control both this creature and another creature and both are unpaired, you may pair this creature with another unpaired creature you control for as long as both remain creatures on the battlefield under your control" and "Whenever another creature enters the battlefield under your control, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control."
702.93b A creature becomes "paired" with another as the result of a soulbond ability. Abilities may refer to a paired creature, the creature another creature is paired with, or whether a creature is paired. An "unpaired" creature is one that is not paired.
702.93c When the soulbond ability resolves, if either object that would be paired is no longer a creature, no longer on the battlefield, or no longer under the control of the player who controls the soulbond ability, neither object becomes paired.
702.93d A creature can be paired with only one other creature.
702.93e A paired creature becomes unpaired if any of the following occur: another player gains control of it or the creature it's paired with; it or the creature it's paired with stops being a creature; or it or the creature it's paired with leaves the battlefield.