I am looking to buy a Zwift hub but I’m not sure if my mountain bike will work.
its a 18 speed with 6 cogs in rear and 3 in front. The size of my cassette size is 28/14.
What style cassette would I get with the Zwift hub that would be compatible with my bike? (8,9,10,11,12)
I don’t really want to buy another bike.
sorry this is all very new to me and I’m a bit overwhelmed trying to find a answer thank you all 
I don’t think there is a 6 speed Hyperglide cassette that you can install on it. 7 speed cassettes are very common and can be put on the Hub with an extra spacer, so at a minimum you would need to get a cassette and replace the right shifter to make it a 7 speed bike.
Your bike most likely has a 6 speed freewheel, not a cassette. 6 speed cassettes are not very common and I think they might all be Uniglide format, not Hyperglide.
It’s generally cheaper to get a new [to you] bike than it is to upgrade the drivetrain
Yeah Ill most likely just buy a different bike thanks for the replies friends