I bought some cheap 120mm led case fans off eBay and connected it to one system fan header using fan splitters. so far it works fine in pwm mode, but in dc mode the fan leds blink like it stops for a sec and turns back on which made me worried if the fans require more power than what the header could safely give so i want to know if it'll damage the header or the motherboard
If in doubt, take some out! Can't tell you if you don't say what amps the sticker/label on the fans says. Or what if any documentation there is of them. The pwm headers are 1amp each so it all depends on what amperage the fans are. I have a silent arctic f12 that says it's 0.08 amps. 10 of those wouldn't be a problem. But the threads I'm reading say you should only have 4 0.18 amp pwm fans per header max.
If they are more amps than 0.18, 5 is one too many. Looks like it's on the margin. Also not sure if that's the default behaviour of the fans or of the pwm mode is lower spd. so the leds light up. Take 4 off, see how one behaves on it's own, add another, see how it is, do the leds blink in dc mode or not?
You can get a molex powered fan hub. I'm feeling the strain.