(Choose any number of permanents and/or players, then give each another counter of each kind already there.) You can choose any permanent that has a counter, including ones controlled by opponents
The number of lands you control is checked as this scheme's triggered ability resolves
Since it counts cards in graveyards regardless of type, it's not uncommon to see this be a massive body on the board
Effects that modify a creature's power and/or toughness without setting it will apply to creatures you control no matter when they started to take effect
While Long.dec above is a pretty perfect example of the kind of Draw-seven deck that has historically been the most common source of turn 1 kills the format was once known for, the most extreme example since the advent of the four-of rule is without question the following Broken LED deck from the 1999 Duelist Team Challenge at Origins
My Aegar, the Freezing Flame deck tries to control the board and draw cards by burning out every little creature my opponents play