I want to make one land that taps for 20+ mana by enchanting the land with cards that let it tap for additional mana. What is the most mana we can get a single land to tap for (obviously excluding a Gaea's Cradle or Serra's Sanctum with 100 creatures/enchantments on board). Also, do the enchantments that have the tap symbol add mana in addition to the lands original production or do they replace the lands original mana production (i.e. Abundant Growth).
I'm assuming Cabal Coffers, Nykthos, Shrine to Nyx and all other similar effects are out. Lotus Vale and its reprint are probably your top contenders for colored mana at 3 mana per tap. If colorless is fine, Scorched Ruins is your go to.
For your rules question, some replace the original ability while others have a triggered mana ability, and some have replacement effects. You'll have to read the card to determine which it is. Abundant growth grants a new tap ability that you may use. Because of this it's relatively useless for this purpose. Remember to apply replacement effects first, then triggers, so with Mana Reflection out, a Scorched Ruins with Fertile Ground on it taps for 8 colorless and 1 mana of any color.
Abundant Growth
For your rules question, some replace the original ability while others have a triggered mana ability, and some have replacement effects. You'll have to read the card to determine which it is. Abundant growth grants a new tap ability that you may use. Because of this it's relatively useless for this purpose. Remember to apply replacement effects first, then triggers, so with Mana Reflection out, a Scorched Ruins with Fertile Ground on it taps for 8 colorless and 1 mana of any color.
Enchantments - Aura
Enchantments - Global
Nonenchantment