Keys can obtained through a variety of ways. This article lists the chances that each key is dropped.
Diamond Keys[edit]
Mining Emerald or Deepslate Emerald Ore: 5%
Mining Diamond or Deepslate Diamond Ore: 2%
Mining Gold or Deepslate Gold Ore: 0.5%
Mining Lapis Lazuli or Deepslate Lapis Lazuli Ore: 0.1%
Mining Redstone or Deepslate Redstone Ore: 0.067%
Mining Iron or Deepslate Iron Ore: 0.05%
Mining Coal or Deepslate Coal Ore: 0.045%
Mining Copper or Deepslate Copper Ore: 0.033%
Mining Nether Quartz or Nether Gold Ore: 0.05%
Spawn Egg Keys[edit]
Fishing: 1.25%
Different Mob Keys[edit]
Taming: 10%
Breeding: 4%
Killing x2, x4, x8, x16, x32, etc: 100%