A good general ratio is to have 23 spells and 17 lands. This strikes a good balance between having enough spells to win with and having enough lands available when you need to cast them. - If you're trying to decide what cards should or should not go in your deck, follow the "B.R.E.A.D." formula. This tells you generally what cards you should be looking to include in a deck, in order from first to last: "bombs", "removal", "evasion", "aggression", "damage". - A "bomb" card is one that has a large and usually immediate effect on the battlefield while having a reasonable mana cost (and power/toughness, if a creature) and few or no drawbacks. It usually does two or more of the other jobs lower on the list. Take, for example, "Flameblast Dragon". It costs 6 mana for a creature with 5 each of power and toughness ("aggression"), it is difficult to block because it has the "flying" ability ("evasion"), and whenever it attacks, you can damage a creature or player of choice ("removal"/"damage"). - A "removal" card is one that can take your opponent's cards off of the battlefield, or at least make it so they aren't threats. This means destroying them, forcing them to be sacrificed, exiling them, or returning them to their owner's hand or deck. Creatures can also be "removed" by damaging them to destruction, lowering their toughness to less than 1, or making it so they can't attack or block (keeping them tapped can also sometimes accomplish this). Prioritize cards that can "remove" creatures, since they are usually the most important cards in that they normally can both attack you and defend your opponent. - An "evasion" card is a creature that has an ability that makes it difficult for your opponent to deal with, or a card that gives a creature such an ability. Usually, this means an ability that makes a creature difficult to block, such as "flying", "fear", "intimidate", "protection", or "can't be blocked". However, it can also mean an ability that makes a creature difficult for your opponent to efficiently get rid of, either in combat ("first strike", "double strike", "deathtouch") or with other spells ("shroud" or "hexproof"). - An "aggression" card is almost always a creature with a reasonable mana cost for its power and toughness, usually costing between 1 and 4 mana. "Reasonable" usually means having power and/or toughness ratings that are equal to or higher than (or occasionally not more than 1 or 2 lower than) its mana cost. Ask yourself: assuming that the creature doesn't have any other abilities, do you think the amount of power and/or toughness it has is worth the amount of mana you are spending to cast it? Then look at its abilities, and ask: do its abilities have a large enough impact on the game to warrant spending extra mana for lower stats? Or, if it seems like you're getting a bargain, does it have a drawback that makes playing it too risky? One final note: be careful about using creatures that cost 1 mana and have only 1 power and toughness each. Unless they have an ability that can have a significant impact on the game (like "Goblin Arsonist" and its "removal" potential or "Doomed Traveler" and its ability to replace itself with a better creature), they usually aren't worth putting in your deck. - A "damage" card is, well, exactly what it sounds like: a card that helps you reduce your opponent's life total, but not much else. Reducing your opponent's life total is usually how you will win games, but creatures and flexible "removal" spells (once that can damage a creature OR a player) are usually more consistent. "Lava Axe" is a good example; it damages your opponent, but it can't eliminate their creatures or give you any of your own. They can be good in an aggressive deck that aims to end games quickly, but usually you will have better options available. - Try to stick to two main colours of magic when building your deck. While using only one colour of magic will increase your deck's consistency, you will usually end up sacrificing power and versatility by playing spells that are not very reliable or have little impact on the game. Conversely, if you try to branch out into too many colours, your deck might be filled with powerful spells, but it will usually be difficult to find the right kinds of lands to cast them with in a timely manner. - With that said, it is sometimes advisable to use a technique called "splashing": constructing your deck with the majority of cards split between two colours of magic, and then including two or three powerful spells of a third colour (along with a few of the appropriate lands needed to cast them). Since you won't be running many lands that provide coloured mana for these spells, though, make sure they don't require too much coloured mana to use. For example, "Doom Blade" and "Assassinate" are both great cards to "splash", since they are powerful "removal" spells and only require one black mana each (the other mana can be any colour). In contrast, "Murder" is also a powerful "removal" spell, but it is difficult to "splash" because it requires two black mana, which is difficult to access reliably if you are only using two or three "Swamp" cards in a deck. - Pay attention to the mana costs of your spells. If you include too many low-mana-cost spells, you will likely sacrifice the power you need to break through your opponent's defenses or deal with their key spells. On the other hand, if you include too many high-mana-cost spells, you run the risk of getting stuck with a hand full of spells that you can't cast, while your opponent plays out their hand and runs you over before you can do much. In general, it's good to have a few spells that cost 1 mana, a large amount that cost 2 or 3 mana, a few that cost 4 mana, even fewer that cost 5 mana, and only one or two that cost 6 mana or more. - Pay attention to the number of coloured mana symbols on the mana costs of your cards; this will help you estimate the correct proportion of different lands that you should use. For example, if the cards in your deck use a total of 17 black mana symbols and 6 white mana symbols, a good balance would be 11 "Swamp" cards and 6 "Plains" cards. Depending on how many cards you have that require multiple mana of a single colour (for example, "Xathrid Demon" requires 6 mana to cast, but at least 3 must be black mana), you may want to tweak this ratio a bit.