Opinions on the number of dark frames varies. Have a look at these sites:
http://www.imagetech-ontario.com/edu...flatbiadrk.htm
http://www.aavso.org/observing/progr...manual/3.shtml
both of which talk about Berry and Burnell's Image-times-five rule. I agree that there are problems with temp control, which is a lot easier with a cooled ccd than a DSLR. A lot of people suggest taking dark frames between each light frame so that things average out.
Geoff