Absolutely, You can run many fans on a single molex conector. Just don't get carried away and exceed the PSU's draw.
Heres some extra info.
To make any 20 or 24 pin PC power supply turn on without a pc, you must bridge the green pin with any black pin. Many people use a paperclip for this, I recomend you simply take a 2" strip of wire, strip 1/4" off both sides of the wire, then use this wire to bridge the green and black pins on the 20 or 24 pin ATX connector.
Once it's on, there are many tricks, to loud a DC fan at 12volts, no problem, you can hard wire it to produce 7 volts uisng the old
7 volt trick
Still to loud or to much CFM? No problem, you can use the
5 volt trick
Lots of computer tricks for growing, for example, I never suffer from electrical noise and my MH and HPS have never shut off due to a power spike or whatnot. I simply have every light plugged into a decent size APC UPS. Been using this method for many years, and it's surprising many others do not do the same. It's far more inexpensive than some 'grow' controllers that do the same thing with blackouts and power spikes and such.
EDIT AND UPDATE: Just some food for thought, but you'd be better served running a Walmart $10 simple AC to DC voltage selectable power adapter. (Selectable 2,4,6,8,10,12 volts) I say this because your wasting electricity running a PC power supply to only run a few fans. If your in the USA, You could also go to
All Electronics Corp - Parts, Supplies and Components or
MPJA and buy a small 12v (or less DC output power supply. If you go to the fan manufacturers site, you could look up the min and max voltage (pay attention to the min voltage to power up the fans, and amperage draw for the fans your using, and if you want a quieter running fan, just get the DC power supply to provide that output.
