There are 23 miracles in total. To use a miracle, equip a talisman in either hand and have the required FTH to assign them to an attunement slot. There are two main merchants in the game who sell miracles, and they are below.

Rhea of Thorolund
Petrus of Thorolund

Merchant Sold Miracles

Great Heal Excerpt - Sold by Petrus/Rhea/Patches.
Great Heal - Sold by Rhea.
Heal - Sold by Petrus/Rhea/Patches.
Homeward - Sold by Petrus/Rhea.
Karmic Justice - Sold by Oswald of Carim. Oswald is the guy who you can request absolution from and he will be located at the bottom of the tower with the Bell of Awakening after ringing it.
Magic Barrier -Sold by Rhea.
Seek Guidance - Sold by Petrus/Rhea.
Wrath of the Gods - Sold by Rhea.
Force - Sold by Petrus/Rhea.
Covenant Related Miracles

Darkmoon Blade - Reach rank 1 in the Darkmoon covenant. See 'Covenant: Blade of the Darkmoon' for how to join this covenant. You must give 10 Souvenirs of Reprisal's. These can be obtained by defeating the guilty world masters or by farming the crow demons in the Painted World of Ariamis.
Gravelord Greatsword Dance - Obtained by reaching rank 1 in the Gravelord Servant covenant. You need 10 Eyes of Death, which can be easily farmed from the basilisks in the Depths or the Great Hollow.
Gravelord Sword Dance - Join the Gravelord Servant covenant. See 'Covenant: Gravelord Servant' for how to do this.
Great Lightning Spear - Reach rank 1 in the Warriors of Sunlight Covenant. You must offer 10 sunlight medals, which can be farmed from the sunlight maggots in the Lost Izalith area or by helping other players successfully kill bosses while you're in the covenant.
Sunlight Spear - After reaching rank 1 in the Warriors of Sunlight covenant, give Gwyn's soul (can only be done on NG+) to the altar and you'll receive this miracle.
Lightning Spear - Join the Warriors of Sunlight covenant. See 'Covenant: Warriors of Sunlight' for how to do this.
Gifted Miracles

Emit Force - Given by Siegmeyer of Catarina after you answer YES to his question. See here for how to follow his story up to that point.
Replenishment - Given by Rhea in the Tomb of the Giants. See here for how to follow her storyline to that point.
Corpse/Chest Loot Miracles

Bountiful Sunlight - *see 'Soothing Sunlight'.
Great Magic Barrier - Treasure from a corpse in Ash Lake.
Inside the hollow tree between the Stone Dragon and the hollow tree from which you entered the area. You must drop from the root walkway surrounding the tree into a hollowed-out tree trunk near the Basilisk.
Soothing Sunlight - This and Bountiful Sunlight are both guaranteed stops from the two non-respawning and non-hostile piasacas that are crying in front of the giant cell at the bottom of the Duke's Archives.
Sunlight Blade - Found on a corpse just past the Gywndolin boss battle. See 'Defeated Dark Sun Gwyndolin' for more details.
Tranquil Walk of Peace - Describing it in text will only confuse you. See this video for the location.
Vow of Silence - Found in the Painted World of Ariamis inside the Annex (requires Annex Key), at the top of building. Some crow demons will ambush you upon picking the miracle up.