What is maximum amount of water changes that I can do?
 

I change 1 gallon a week in my 14 gallon reef. Can I change out more water? Like maybe a 1 gallon water change twice a week or am I at the maximum I can change with just 1 gallon a week?
Suggestions on a good water change routine is appreciated, thanks!

I personally like to change 10% or less at time, but doing multiple 10% changes over the course of a week is not a problem.

I agree, the water in the ocean is technically changed and i quote from Eric Boneman "I would add that the coral reef gets a 100% water change 2-3 times per day!" in Reef Food by Eric Borneman - Reefkeeping.com for ref.

Do I calculate the 10% by the tank size (14 gallons) or by the actual amount of water in the tank (which is actually around 7-8 gallons)?

I'd go with the median so 1 gallon. You would normally just go with the water volume, but i don't see 1gallon an issue.