Bathroom ventilation - specifically air changes per hour

[FraserB]

I am looking to install a loft mounted extractor fan to prevent the bathroom from steaming up when the shower is running. After reading various articles there seems to be a difference between some who say 6 to 8 air changes per hour are recommended and some who say 15 to 20 are recommended.
This is for a standard domestic bathroom which starts to run with condensation when the shower is on. What rate of air changes per hour should I require?
Also but less important is how do you know how noisy it will be when only dbm references are supplied which don't mean anything to me?

 

[foxhole]

The bigger the better, [no such thing as a standard domestic bathroom they can be a few cubic meter's to dozens] I use a 6 inch fan in the loft suspended on springs, this reduces any noise transfer to timber and therefor to the ceiling.
This is connected to twin 4inch vents in the shower ceiling.The more efficient the fan the louder it tends to be, you need to decide on you priority.I doubt you will have the fan on for more than a few minutes at a time so does the noise really matter?.[My shower pump make more noise than the fan.]