why do you start with seven cards in magic like why did they decide on that number
why do you start with seven cards in magic like why did they decide on that number
quite simple ... when you design a game there are a lot of numbers you have to decide on. how you go about deciding on those numbers depends on how you design games.
there is the intuitive aproche where you just pick the number you feel might work best and then test it.
what more likely happend with magic is the mathematical aproach. yes math. the subject almost everybody hates in school. game design is math all the way. there is a whole branche of math called game theory.
now richard garfield, the guy who inveted the game, is a ph.d. in math. (Richard Garfield, Ph.D. ^^). so there will have been some consideration based on statistics, interaction with the cardnumber in your deck, probabal land / non-land ratio and so forth.
the last step to deciding if the number is right will be endless hours of playtesting though. basicly you play game after game and see what works and what does not. what leads to more fun games and what does not.
7 starting cards is the result of a combination of clever thinking, intuition, math and a lot of work (and yes playing games can be work ... trust me i have playtested other games befor ...)
hope that gives you some insights.