how can you tell by looking at the motherboard how many fans you can install?

You can't..really. Even though you may have a certain number of fan "headers" - you could add more by splitting the power from these. How many do you need to add?

basically i wanted 4 fans (3 fans on the case and 1 for cooling the CPU)
the motherboard i want is Gigabyte GA-P35C-DS3R (dont know how good it is for the mild user)

the website Techgage - Gigabyte GA-P35C-DS3R says that one of the bad things about this board is that it has a lack of (only 3) fan controllers (so thats 1 for CPU and 2 extra)
how would i split it as you said?

Well, if you get fans that are molex connected, then something like this would do for you to double the plugs: Wye Power Cable Splitter Molex 4-pin

oh right....is the voltage to each fan halved as well?

is the molex what OC's use to cool their case with multiple fans?

No but every time you share two devices on a single connector, all the power drawn by the two devices has to travel down the same set of wires from the power supply - this could become a single source of failure inside your PC. Also, if you do buy one of these, make sure it is made well and also make sure that all the connections fit snugly when connecting inside your PC case.

No, the molex is the type of plug that connects devices to the PSU

My advice would be to use the on-board fan headers and if you need another one fan, get a molex connected one and don't use a Y - splitter

Probably no need for 4 case fans... the reason I say this is that cooling is more about air-flow.
This can be achieved (usually) with one front case fan and the CPU fan drawing cool air into the case-- this then circulates throughout the hardware... picks up the heat-- and then exits via 2 rear exhaust fans-- this way you have a steady, unobstructed flow of air passing through constantly.
Noise is another consideration-- Generally, the smaller fans are the noisiest
Larger diameter fans move more air with less racket. Noise also varies by brand (quite a bit)
Internal cabling is also a factor--rat's-nests of wires everywhere greatly reduces efficiency. Neatness is good.

Some good sites for cooling are:

Quiet Computers - End PC Noise
silentpcreview.com | Everything about Silent / Quiet Computers

The reason I recommend you consider noise is that after a long while-- the constant whine of high-RPM fans gets really annoying.