The case comes with 3 RGB fans and I will buy 3 more MasterFan Pro 120 RGB. That is 6 RGB fans + the Cooler Master RGB Lighting LED Strip. How many RGB headers would I need on my motherboard to support this? I'm guessing I can use some kind of splitter like this: https://www.amazon.com/dp/B072K21JVF so could I plug 4 fans into 1 RGB header and the other 2 fans + RGB strip into the other?
Yes you can but you want to take into consideration the amperage of the total load and the capacity of the headers as you do not want to exceed that. If you require more than what is capable via the header there are power RGB splitters which are designed to accommodate more RGB components.