You can safely break as many Demon/Crimson Altars as you need to progress, but most players stop after a small handful because of diminishing returns and convenience.

What breaking altars does

  • Destroying an altar lets Hardmode ores (Cobalt/Palladium → Mythril/Orichalcum → Adamantite/Titanium) begin to appear in the world and produces one or more patches of those ores; the first few altars give the largest ore yields.
  • In modern Terraria (post‑1.4 updates) altar destruction no longer permanently spreads Corruption/Crimson/Hallow across large areas the way old guides described, so you won't ruin the world by breaking many altars.
  • Each altar destroyed also triggers enemies (wraiths or a brief attack) when broken, but this is only a short nuisance rather than a permanent penalty.

Practical numbers players use

  • 3 altars: a common minimum — guarantees at least one full cycle of each ore tier and is usually enough to get started with Hardmode progression.
  • 6–9 altars: a common “sweet spot” many players prefer because it supplies noticeably more ore without much extra hassle.
  • 10+ altars (even tens like 50): technically possible and safe in modern versions; after the first several sets the additional ore you get becomes very minor, so this is usually unnecessary unless you want every last tiny patch.

Recommended approach (step‑by‑step)

  1. Break 3 altars first to unlock all three tiers of Hardmode ores and see what your world generates.
  1. If you find you’re short on a specific ore (especially Adamantite/Titanium), break additional altars in small batches (3 at a time) until you locate more deposits.
  1. Keep at least one altar intact if you want to craft certain pre‑Hardmode boss items or for redundancy.

Example scenario

  • If you want a quick, efficient start: break 3 altars, collect ore, then break 3 more later if necessary — that gives a balanced supply without chasing tiny, marginal patches.

Final tip

Because returns diminish after the early altar cycles, focus on exploring cavern ore veins, fishing crates, and world generation patches as alternatives if you need a lot of a single ore type.

Information gathered from public forums and the Terraria community.