The more the better.
Many wargames use a lot of standard six-siders. You roll handfuls at a time to smite your opponent.
With RPGs you can never have enough polyhedrons: d3s, d4s, d5s, d6s, d7s, d8s, d10s, d12s, d14s, d16s, d20s, d24s, d30s, d100s.
You need a wide selection in case your luck runs out. If one die rolls bad, you can pick another die.
If “1” is the worst result on a die, to jinx your opponent, you say, “Roll anything but a ‘1’.”
(This works in wargames fairly well, but you can use it to confound your GM in RPGs).
We knew a D&D player who had a trusty d20 which failed him during an epic fight against a horde of orcs and goblins.
It rolled a “1” four times in a row, a critical failure each time. Angry, the player picked up the d20 and hurled it across the room. It struck the wall, bounced on the carpet, rolled back toward the table for all to see.
It came up with yet another “1.”
It was time for that d20 to go.
To undo any jinxes and bring good fortune, we blow on our dice before we roll them.
Personal sets of dice are kept separate from ones that may be borrowed.
Never let cursed dice touch blessed ones.
Roll those dice, no take-backs.
Alea iacta est!