I would say that the more the merrier if you like good handling. But, the front ones do most of the improvement. The rear is more important if you're hauling a load. I have lowered my 64 C20, and added front and rear sway bars, for the best possible handling.
Here are a couple of articles from Early Classic that show the results.
http://www.earlyclassic.com/Tech.aspx?category=Mid+60's+Makeover+I
http://www.earlyclassic.com/Tech.aspx?category=Mid+60's+Makeover+II
