For me it's pretty simple, because my farms are situated in a relatively cool climate (temperatures are rarely over 30 deg here in summer) - and most of the year in the 10-20 degrees range. I just made sure that my extractor fans were capable of moving more air than the miners would exhaust. Obviously it's important to ensure adequate intake also.

For 17 / 19-series Bitmain, expect up to 500cfm of expelled air per miner (credit @philipma1957).

So if you're running 20 of the above, your exhaust fans need to be capable of moving 10,000cfm (20 x 500). You should consider also that the exhaust fans may lose efficiency over time (e.g. as dust builds up on the blades) so it would be sensible practice to allow a margin on top of this (so 10,000cfm + 15% = 11,500cfm for 20 miners).

Depending on the country, the fan capacity may be quoted differently e.g. meters per minute or meters per hour - but there are plenty of calculators and formulas online to convert into cfm.