At the top of the description on the 32 oz Cleansing Conditioners (A238707) there's a .pdf on "How Much to Use", the link is http://www.qvc.com/footers/hb/pdf/WEN_PumpChart_03012013.pdf. Also, in the FAQs, there's a question about how many pumps per bottle - a 16 oz bottle has approx. 160 pumps, a 32 oz has approx. 192 pumps. The reason there's not double the number of pumps between a 16 oz and 32 oz is the 32 oz pump has a longer stroke and a wider tube so more comes out for each pump.
If you are wanting to be sure, pump into a measuring cup and you can see what a full pump measures or what the amount of pumps you use looks like. There was a thread a while ago about how many WENners counted pumps and how many "glopped" or went by feel - think it ended up about 50-50. As Kjelle said, it's about experimentation - after you've used WEN a while, you just get a feel for what feels right in your hand and in your hair.
The amount I use varies depending on how long it's been since I last cleansed, whether I am cleansing out an oil treatment, or just how my hair feels that day.