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Only one fan is needed. I list a variety of fans so that you can choose the best one for your application. The major difference is Airflow vs Noise. I chose the highest airflow for my application because the printer lives in my workshop. You might want a very quiet one if your printer is located in a quiet space. Below is a copy of the fans in the description:

Fan      Part Number      RPM   CFM  Noise Length
Noctura  NF-A6x25 FLX     3000   17  19Dba 25mm
Sunon    PMD1206PMB3-A    6000   42  47Dba 38mm
San Ace  9G0612G101       11,800 65  58Dba 38mm