The "CFM" number is the Maximum air flow a fan can deliver when blowing against almost no resistance or "back-pressure". More air flow is better, although that usually means also more noise. However, many fan designs can deliver more flow at much less noise than some fans.
The "mm H2O" number is the maximum back-pressure the fan can blow against, and at that back-pressure the actual air flow has dropped to almost nothing. If you sketch out a graph of air flow in CFM versus mm H2O back-pressure, the line from max flow at no back-pressure to no flow at max pressure is ROUGHLY a straight line for any given fan. So, different sketches for different fans will allow you to compare the important item, Air Flow, at the same back-pressures.
Now, at what back-pressures? A fan with nothing in front of it or behind it experiences zero back-pressure and can deliver its max flow. A fan with a foam dust filter on its suction side but free space at the discharge side (typical of a case ventilation fan) experiences small back-pressure and can deliver most of its air flow. A fan on a finned heatsink for a CPU experiences significant back-pressure and will deliver less air flow that its max rating. There are MANY fan designs specified as "high pressure", meaning that they can still deliver good air flow (not their max rating) at significant back-pressures, such as when they are mounted to cool a CPU heatsink. These fans, however, are not ideal for case fans because typically they do not deliver as much air flow as a "low-pressure" or "air flow" fan when used in low-back-pressure mountings.
You have not specified what mobo you have, nor how many fans you have for case ventilation, not the fan type - that is, 3-pin or 4-pin. These are also important factors.
3-pin and 4-pin fans require different methods of control by the mobo fan header. Sometimes it is hard to tell what method a header uses, but if you post back here the maker and exact model number of your mob, we can tell you which fan type (maybe either) you can use and still be able to control them properly.
A common mobo fan header can supply up to 1.0 amps to the fan(s) connected to it. You may be able to use a Splitter or a Hub to connect more than one fan to a header. Splitters and Hubs are two different types of devices, so if you plan to use something to connect several fans to a header, tell us and we can provide more details.
If you look up the specs of the fans you are considering, each should tell you the max current it uses. Most common fans consume at max 0.1 to 0.3 amps, so using 2 or 3 on a single mobo header is OK. BUT fans that include LED's (as you want) use MORE current, so watch for that carefully. A few fancy LED fans are different because they power and control the LED's separately, but most do not do that. However, it appears the ones you liked to, the Cooler Master MasterFan Pro LED line, does just that. FRom their website I gather the fans each have two connection cables - one fro the fan that plugs into a mobo 4-pin header, and another that must connect to a LED controller. That latter controller may be one that Cooler Master sells for this purpose, OR it may be a LED control system built into your mobo.
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