Cake Pops: Ratio of cake crumbs to frosting?
Cake pops all depend on moisture of the baked or scrap cake you are starting with (and about a hundred other factors including ambient temperature and humidity, fat %, age of cake, etc etc etc).

The best way to determine your ratio is to first start with your cake in a stand mixer (if you have one) with a paddle attachment. Mix the pieces of cake on low speed and slowly work up to medium speed. Once it's all crumbed and uniform, grab a handful or one cake pop worth of cake and give it a squeeze. Does it form a ball on its own? If it's close to forming as is, add very little frosting/icing/buttercream. If it's still loose and crumbly, add a small amount of your frosting/icing/buttercream and continue mixing and test the batch again by squeezing a portion. 

A little trick: to help in binding the mixture and to incorporate the sweet component of the icing (without adding so much icing it becomes too loose), add a good dusting of icing sugar before the icing while you are mixing/crumb-ing. 

Hope that helps, cake pops can be tricky sometimes, epsecially with very fresh cake or very stale/hard edged cake. There's no specific fool-proof formula for these.