Just a reminder, my set-up is far from perfect, however, after all these years I have come to realize that heat isn't really the main reason these gears get toasted or become useless, they either become obsolete due to profitability or fail due to bad "luck" or quality if I may call it. if you look at my topic which nc50lc quoted, I have two farms (currently building a new one), one had a very poor airflow, miners did run pretty hot almost all summer long, but the effect of that on the lifespan of those miners was almost NOTHING.
Add to that stories I have heard from various miners, some people who had a mediocre cooling reported less failure rate compared to those running their miners in high-quality data centers, I know many people who "go by the book" will criticize me for saying this, and they have the upper hand because their point is "scientific-based", I am telling you this because I believe you shouldn't go to the extreme about airflow.
Now the numbers:
Phill
thinks that S17 move about 500cfm of air, assuming you have 5 of them, you will need to bring in 2500cfm and exhaust a little more than that, later on, if you think the airflow is not sufficient, you can add more fans.