There are a total of 12 Miracles to be learned in this game. You can get them all in one playthrough of the game, provided you don't spend the required Demon Souls on anything else (or consume them).
Before you can learn any, you'll need a Faith stat of at least 10. Once you have so, talk to the Disciple of God in the Nexus and you can buy the following four Miracles:
• Antidote (3,000 Souls)
• Evacuate (20,000 Souls)
• Heal (5,000 Souls)
• Hidden Soul (3,000 Souls)
To unlock the rest, you'll need to first save Saint Urbain. To do this, go to World 4-2 (Shrine of Storms) and follow the Augite of Guidance (the shining stones) on the floor at the rear of the room with the Reaper in it, to the right. These will lead you to Patches, the Hyena, who will kick you down a hole and trap you in a room with Urbain and a black phantom. Talk to Urbain if you want, then kill the black phantom (he's an easy opponent) and then talk to Urbain again. Then head upstairs and step on the switch to emerge behind Patches. He'll apologise and give you a Ring of Gash Resistance, at which point you can leave.
When you return to the Nexus, Patches and Urbain will be there. Urbain will replace the Disciple of God and will sell you all the Miracles in the game. The following seven Miracles can be bought with the Souls you get for defeating their respective Demon:
• God's Wrath (Dragon Demon's Soul [2-3 Dragon God])
• Banish (Yellow Demon's Soul [3-3 Old Monk])
• Regeneration (Swollen Demon's Soul [4-1 Adjudicator])
• Second Chance (Hero Demon's Soul [4-2 Old Hero])
• Anti-Magic Field (Storm Demon's Soul [4-3 Storm King])
• Cure (Wriggling Demon's Soul [5-1 Leechmonger])
• Resurrection (Pureblood Demon's Soul [5-3 Maiden Astraea])
The final Miracle is slightly tricky: it costs 3 Colorless Demon's Souls to buy, but you must also have defeated a Primeval Demon for it to become available for purchase.
• Recovery (3 Colorless Demon's Souls)
You can trade a Gold Mask and a Talisman of God to Sparkly, the Crow in World 4-1 for two Colorless Demon's Souls. To trade them, drop them on the floor in front of the tree. If Sparkly approves, quit the game and load it again to find the Colorless Demon's Soul pickup to the right of the tree. Make sure to only trade one item at a time. A third Colorless Demon's Soul is yours if you go to World 1-1 with Pure Black or Pure White World Tendency and go through the open door to the left at the start. In the tower, go down the stairs and drop onto a wooden beam near the ladder. You can drop onto another beam underneath and pick up the Colorless Demon's Soul from the corpse here, but if you misstep, you will die, so be careful.
To kill a Primeval Demon quickly, go to World 2-1 and attain Pure Black World Tendency (keep using Stone of Ephemeral Eyes to revive and suicide off the edge near the beginning. You'll need to return to the Nexus and come back to see any changes in World Tendency, which can be checked in the menu). Once you have this required World Tendency, cross the bridge where the two Scale Miners are hurling boulders and go to the right. Pull the switch and take the lift down: at Pure Black World Tendency, a Primeval Demon will be there – you can't miss it. There's a high chance it will drop a Colorless Demon's Soul itself, and killing it will finally allow you to buy Recovery.
Note that aside from this trophy, Miracles are largely based on personal preference; Evacuate and Second Chance are the only Miracles that I'd highly recommend, and even then Second Chance is only useful for those who plan on starting New Game+.