There are 25 Shrine Locations in Shadow of the Colossus PS4 Remake. You must pray at all shrines for trophy “Seeking Salvation”.
One of the shrines is only accessible after beating Colossus #15, on your way to Colossus #16 after lighting your sword to open a gate. At this point you can still go back to get all the other shrines.
Go past this point and it will be impossible to go back (only with an old backup save). This makes the shrines missable if you go too far. You can get all other shrines at any time, even right after the game starts. Just this one shrine is locked behind the gate on the way to Colossus #16 (it is circled in red on the map below).
This is what a shrine looks like:
They are big stone pillars scattered around the open world, very easy to see. To pray at them, walk towards the glowing center stone and hold . This creates an autosave. Basically, they are just save points. In case you die you will respawn there and don’t have to backtrack all the way from the main shrine.
Shrine Locations Map
Below is the map with all 25 shrine locations. The last one that unlocks after Colossus #15 is circled in red. All others are available at any time from the start of the game. The shrines have yellow icons on the map.