how the list is tracked
Achievements live on a panel inside the game rather than on a storefront service, and the counting entries do not reset when you start over. The Angler quest achievements, for example, tally every quest turned in by every character across every world you have played, so an abandoned save still contributes. That makes the grind entries far less painful than they look on first reading, because nothing you have already done is thrown away by rolling a new world.
how many there are
The current total is 137, and they arrived in waves rather than all at once: 88 with 1.3.0.1, one more in 1.4.0.1, then fifteen in 1.4.1, eleven in 1.4.4 and twenty two in 1.4.5.0. If you are returning to an old character, expect a burst of unlocks in the first hour simply from things you did years ago, since each batch was written around the content shipping beside it.
one kill can close several at once
Nothing stops a single event from satisfying several achievements together, and the overlap is worth planning if you want the list finished. King Slime can complete five in one death if you save him for last and finish him from a minecart. Any mechanical boss can close four the same way, and Pinky can manage three. The pattern is always the same: line up the boss entry, the collection entry and the how you killed it entry so they resolve on the same corpse.
the ones with a trick
- Completely Awesome only asks you to own a Minishark. If the gun itself does not interest you, buy it from the Arms Dealer and sell it straight back without closing his shop window. The unlock stays and the money comes back in full.
- Journey worlds unlock everything normally except Extra Life, which requires surviving a death as a Hardcore character. That one needs a separate save, and a specific seed combination, to be reachable at all.
achievements that were written but never turned on
Six entries sit in the game code with names and descriptions but no way to earn them. Behind The Mask would have covered the Insane Cultist and was cut because a near identical goal already existed. Davy Jones Locker points at the Flying Dutchman, Winterhearted at the Ice Queen, Pumpkin Smasher at the Pumpking, Independence Day at a Martian Mothership, and Hex Education at the Goblin Warlock. The last name is a pun aimed at the hex weapons he carries, the Shadowflame Hex Doll and the Shadowflame Knife.
the second list on some platforms
A few platforms carry an older achievement set alongside the in game one. The Xbox One, iOS and Google Play builds inherited lists from the old console version and from their respective storefronts, and none of those are maintained any more. When an external tracker disagrees with the panel in the game, the panel is the one to believe.