For my build I will be using an i5 CPU and a gtx 760. My case is a - Corsair CC-9011050-WW Carbide Series SPEC-01 Mid-Tower ATX Gaming Case. This case comes with a fan but I'm not sure how many I will need. Thanks.
Depending on the temperatures you get, if you have overcloking/overvolt the cpu, if the 760 is of the blower type and exit outiside the case teh hot air, etc....
As rule of the tumb i would say three, one in the front who bring fresh air into the case and two out, one back and one top to get rid of the hot air inside the case.