I've mentioned a couple of times that I am using Arctic fans, partly because there is a pretty good price for a 5 pack. For my house, I'm making 5 Ventbots with 3 fans each. That works out to 3 boxes of Arcitc 5 packs. That's how I know I have exactly 15 Arctic fans. Except, now that I am assembling fans into brackets, I can only only locate 14 of them. I can think of no explanation other than someone breaking into my house and taking it away from me (while ignoring all my other treasures) in an attempt to drive me crazy. In place of that 15th Arctic fan, I'm using a be quiet! fan that I bought for testing. It's actually just a tick quieter than the Arctic fans by my own measurement. They're also a bit more expensive than the Arctics.
Incidentally, the exact model of Arctic fan, the Arctic F9 PWM PST, has been discontinued. The similar Arctic F9 PWM is still available. The difference, as far as I can see, is just the connecting cable. The PST model had a cable to chaining multiple fans together and controlling them all with a single PWM signal. I'm not using that feature, and I originally bought that model because it was the same price as the F9 PWM with a more typical cable. I was just keeping my options open.