I want to install new brakes how do I know if its 1 or four piston?

 

start with the model. Do yu know what model you have?

If not, then you will have to look at the front brakes themselves. The 4 piston calipers will have Mazda in raised lettering on the caliper seen from the outside.

 

4-stud wheels = single piston
5-stud wheels = 4-piston

 

well we had the base S5 models with 5 lugs and a single piston on the front... so that wokrs for the S4 models, but not the S5 models.

 

do all fc's have vented front disks?

 

fronts - yes
rears - optional

 

Let's also point out the obvious: Someone who can't immediately tell the number of pistons by looking at their calipers should not be installing brakes.