While the world of RPGs thrives on many different types of dice, the six-sided die remains at the core of many different role-playing games. Since dice manufacturers started specializing in gaming dice, they have been making 12d6 dice sets, giving gamers an easy way to acquire large dice pools of this high-use die. The 12d6 set contains 16mm dice, the standard dice size for the majority of plastic dice.
It's worth noting that it's traditional for 12d6 dice sets to use pips rather than written numbers. The pips on these dice are the dots you'll commonly see on dice used for mass market board games like Monopoly. The dice within 12d6 dice sets are slightly different from those in a 36d6 set. 13d6 dice sets contain smaller, 12mm dice.
12d6 dice sets are commonly used in games like Shadowrun and Champions, along with any other RPGs. In games that use large quantities of d6 dice, it is possible for well-optimized characters to make use of more than a dozen in a single roll! The traditional solution to this problem is, of course, another 12d6 dice set! Two is as many as most will need for the majority of games, though there are some situations where you may want even more. Of course, you can always just reroll a few dice or borrow some from your neighbor (in this case, the person sitting next to you at the table. The guy next door is probably going to be confused if you ring the bell asking for a half cup of d6s).