Confusion.
For relay approach you need one BC337, one relay, one diode, independently how many fans you have.
For transistor option you need BC337 plus 2-8 BC327 for 3-15 fans.
For mosfet option you need one BC337, one P-channel mosfet, but the mosfet needs to have characteristics to saturate well with -12V gate voltage.
Confusion indeed 
Can you help me on setting this up?
Also this looks ok.
No problem.
Awesome. Can you tell me which relay and diode I need? I’ll do my best to set it up and come back with that!
Whatever 12V >8A spst (or spdt) relay is fine. Why 8A? I like to have 3x margin for relays that are not rated for inductive load.
https://www.aliexpress.com/item/33037376542.html
https://www.aliexpress.com/item/1005003128290617.html
Obviously some brand like Omron offers more reliability, but costs 5 times more (or is fake).
Also whatever diode rated >20V/200ma, for example the same 1N4007 you use for flyback.