hydrotech364, 80mm PC fan needs around 0.15-0.20Amp. A 12V Charger will output from 0.5 to 1 Amp depending on the model, that why you should read the labels
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Knowing that, you can make a quick math - how many fans you can hook to the single power adapter. This setup will last you as long as using PC power supply but in much simpler, cleaner and less cumbersome setup.
And don't forget that you need to short 2 pins on PC power supply to make it work at the first, which I would not call very safe.