Also, is there a way to find the average number of peers that these sites multihome with? If not, how large is it in general?
Difficult to say, and lots of people have tried. Route-Views @ Oregaon, CAIDA, RIPE RIS, and many others has some data you might be able to morph into that.
The first problem is to identify what defines a site. Is an ASN a site? Would all of AS701 be considered a single site? If not, then, how can one identify a site. Is a prefix a site? What about prefixes that are aggregated and span multiple locations all over the world? There is no clear correlation between routing data and any logical definition of the term site. Is a site a building, a campus, the collection of buildings networked in a given metro. area? There isn't even a clear definition of the term site, really. If you can define what it is you want to measure, then, perhaps it can be measured. However, the question as stated cannot be measured because the terms used are not sufficiently defined as to allow measurably correct meaning. Owen