I am taking 12V from Power card to my backplane card to use for Hard disks. I have been advised to put decaps of 330 uF on 12V supply. Can I use 220 uF or even lesser value. What is the criteria for selcting the value 7 what other parameters should I check??
Unfortunately, there only correct answer to this is "read the datasheet". Lacking a datasheet, you need to have knowledge of the current draw characteristics of the specific hard disks you're using. Lacking that, you're left with trial-and-error.
Assuming you're in the trial-and-error boat, put the 220uF capacitor on there and see if it works. If you get strange behavior, increase the capacitance. You can't really go too high on a decoupling capacitor, so it's safer to err on the high side.