How many arms do I have to replace, I thought there are only 2 lower arms?

 

there are 2 lower arms and 2 upper arms, most of the time you are trying to remove negative camber and only need to replace the lower arms.

So depending on why you are changing your arms it could be 2 or 4, I only did the lower arms on mine and I think that most people only do the lowers.

 

You only need to replace the lower arms to make camber adj. and toe is adj. at the front of the trailing arm but replacing both pairs of arms will make your toe adjustable at the arms as well and most alignment places would rather do it there, as it's easier to make fine adjustments and that's assumeing they can even find the stock adjustment point which requires loosening 6 bolts prying, measuring, prying , measuring, prying, measuring ect. Also all the after market arms are stronger than the oem arms and by having four instead of two you will be able to change your rear track, move both wheels in or out. Many people with tall tires, wide tires or wheels with non stock offsets that have rear rubbing could benefit from that. To be autocross legal you need to get arms like the H-Sports though and they all look better.