What is the average number of publications (including conference proceedings), per year, for CS postdoctoral researchers in the US? What are the ways to find out or estimate this number?
The main motive behind this question is to understand where am I standing with my own performance and productivity.
Then you're asking the wrong question. The right metric to examine isn't the number of publication, but rather your visibility and reputation (and ultimately impact) within the research community. If the intellectual leaders of your target subcommunity are eager to write you good recommendation letters, it doesn't matter whether you publish one paper a year or ten. Similarly, if the intellectual leaders in your target subcommunity are not eager to write you good recommendation letters, it doesn't matter whether you publish one paper a year or ten.
I'm guessing nobody really knows this statistic, especially in computer science where it's not clear what counts as a publication. Are all conference papers actually "publications," even in non-selective venues? Do all workshops count or only the selective ones? Do journal papers count in addition to their conference counterparts? What about letters papers? And where does arXiv stand?
Even worse, there's no good tool for you to get an estimate. For example, in machine learning, two of the top conferences (NIPS and AISTATS) aren't always represented in DBLP. The best is probably to visit the websites of other postdocs in fields similar to your own and see what they've done.
I'm also a postdoc, so I might not have have the correct perspective, but I also think that the number of papers doesn't really matter; what matters is the quality, consistency, and impact of your work.
Finally, "where you stand" relative to your competition depends on what job you'd like. My feeling is that, in general, people who get research positions at good labs or faculty positions at research universities probably have at least a couple publications in the top venues of their field per postdoc (and/or end-of-Ph.D.) year.