When enchanting a tapped land using New Horizons can I still produce mana using it or do I have to wait for my next turn?
Knowing that the enchantment let's you produce 2 mana when tapping the enchanted land, since I enchanted a tapped land that already produced 1 mana, but can I produce 1 more mana after enchanted that tapped land that already produced 1 mana?
I'm not sure if can't produce any mana, produce 1 mana, or produce 2 mana.
You need to wait until the land untaps to get more mana from it.
Lands generally have abilities that allow you to tap them to produce mana. New Horizons gives the land a different ability that also allows you to tap it to produce mana. If the land is already tapped, you can't use any abilities that require you to tap it until it untaps.
Basic lands (Plains, Island, Swamp, Mountain, Forest) and other lands with basic land types automatically have an ability "{T}: Add {M}" where M is the corresponding color of mana, as a result of rule 305.6. Most other lands explicitly have similar abilities printed on them.
You will need to wait until its untapped to get more mana from it.
You seem to be under the misconception that Basic Lands give mana when tapped. This is not accurate. Basic Lands give mana as the result of using their intrinsic activated ability. Using this ability requires tapping the land, but it's not tapping the land that gives the mana, it's activating the ability. For example, using Twiddle to tap a Basic Land would not generate mana.
In your scenario, the ability that granted the mana has long since resolved. There's nothing you can do now that will change the amount of mana it gave.
And the newly granted ability won't produce any mana until its activated. To activate the new ability, you need to tap the land, which you can't do if it's already tapped.
Either way, the new ability will never grant just one mana. The instructions clearly say you add two mana to your mana pool, not just one.
