The standard RPG 7-dice set includes all of the standard polyhedral dice. Specifically, they include the following:
- 4-sided die
- 6-sided die
- 8-sided die
- 10-sided die
- 10-sided percentiles die (for use paired with a d10 to give percentile, or d100 results)
- 12-sided die
- 20-sided die
These RPG dice sets are the staple of the gaming world and are used in games like D&D, Pathfinder, Earthdawn, any d20 game, Call of Cthulhu, and many more. Also, a 7-dice set covers all of the dice you need for these games. However, it only gives you one of each of these dice. Therefore, if your wizard is launching a fireball at a mass of orcs the 7-dice set has you covered -- it has the d6 for your fireball damage and you can just roll it over and over. But most gamers prefer to have a larger number of some of the die types that you more commonly need to roll in clusters (d6 is probably the one that you need the most).
There are a few ways to build up multiples of these dice -- you could purchase them individually (which is kind of a pain) -- but more commonly gamers prefer to get multiple RPG dice sets in different colors or styles, which allows us to use different dice sets for different games or characters, or to lend out dice to a player who forgot theirs without any danger of confusing whose dice are whose. So grabbing a few different 7-dice sets serves this function well. Alternatively, you could grab a 12d6 set, since six-sided dice are one of those that you commonly need a lot of, and then a couple of 7-dice sets. Then there's the killer Pound o' Dice solution -- just getting a mass of different dice to fill in for all the occasions when you need MOAR DICE!
Of course, here at Awesome Dice, our preferred solution is to do all of them. After all, a gamer can never have enough RPG dice.