Slay the Spire has 3 main acts , plus an optional Act 4 as a bonus final challenge.

Quick Scoop

  • Acts 1–3 are the standard run structure.
  • Act 4 unlocks only after you collect the three keys and beat Act 3.
  • So the short answer is: 3 acts in a normal run, 4 total if you unlock the secret final act.

TL;DR: 3 main acts, 4 with Act 4 unlocked.