some come with 1, other with 2, some with 3 and most come with none
what case are you buying?
specs of your machine, the one you plan to build
any case in mind?
if you have some cases in mind, you can visit the manufacture website for the case, it will tell you how many fans and where you can install them and if it will give you any of them preinstalled
some cases will give you fans but when you turn the case on, you will hear how noisy they are and you will feel the need to remove them and buy better fans