Horizon Forbidden West - All Relic Ruins Guide

You can find 9 Relic Ruins in Horizon: Fw, but you only need to complete 3 to unlock Completed 3 Relic Ruins trophy. You can find 8 of them on the map, and the location of the 9th and the final one is unlocked during side Quest "Nights of Lights" (started by talking to Stemmur inside the Hidden Ember settlement).

It's advisable that you finish most of the story missions to unlock all needed gadgets, otherwise you will be blocked from progressing through some of the Relic Ruins.

Relic Runs: The Daunt

Security code for locked doors is 1705.

Relic Runs: No Man's Land

Security code for locked doors is 2204.

Relic Runs: Restless Weald

Security code for locked doors is 1923.

Relic Runs: The Dry Yearn

No security code here.

Relic Runs: The Stillsands

No security code here.

Relic Runs: Runner's Wild

No security code here.

Relic Runs: The Long Coast

No security code here.

Relic Runs: Isle of Spires

Security code for locked doors is 2109109 (2109 & 109 found from data holograms).

Relic Runs: Errand Quest - Nights of Lights

The location of the 9th and the final one is unlocked during side Quest "Nights of Lights" (started by talking to Stemmur inside the Hidden Ember settlement). That will unlock after clearing the "The Sea of Sands" main story quest. 

Security code for locked doors is 739135 (135 & 739  found from data holograms).