What’s the largest number of sides you can have on a die without it 1) becoming the size that makes it cumbersome to use, and 2) prevents it from rolling forever due to it approaching the shape of a sphere, and 3) losing symmetry? The answer, according to The Dice Lab is a 120 sided die . . . a d120.
Like most dice with high count faces, the sides add up to N+1, where N is the highest number you can roll on the die. For example, all d6 opposite faces add up to 7. In this case opposite faces add up to 121.
A single die will cost you $12 whereas a set of five sets you back just $50. Definitely spendy, but I’m assuming there was significant expense involved with the plastic injection tooling needed to get 120 faces in a reasonably small space.
I’m not sure what use you would have for a d120, since most RPG systems max out at 100 because percentage is a natural part of any system. But the geek in you may just want a d12 for your collection.