How complete are academic databases? Right now I'm using Web of Knowledge, ScienceDirect, etc. to find papers to read and get some metadata. My university has a subscription, so this is nice. As far as I know these databases simply have a selection of journals, download all of the articles from those journals, and display them for me, updating every time the journal publishes. My question is: what percentage of journals are actually represented here? Some people say there are between 10,000,000 and 100,000,000 published academic documents in the sciences. Web of Science has about 50,000,000, and it's not clear how many ScienceDirect has.

A good comparison would be: if you take everything that Elsevier, Wiley, Springer, and Nature have, combined, and subtract everything that Web of Knowledge has, how many things are left?

Another way to phrase it: Of the major publishers (like the ones I listed above), how many are represented in these databases?
The precise extent of scholarly publishing is unknown, difficult to estimate, and the subject of some debate. That said, there is lots of evidence that Web of Knowledge and ScienceDirect are likely only the tip of the iceberg.

That is sort of the takeaway from an article I found in from Library Journal on Online Databases-Online Scholarly Journals: How Many? (Google WebCache because the LJ copy seems gone) by Carol Tenopir and published in February 2004.

Apparently, one of the best sources online is Ulrichsweb which is a serials tracking website that is part of ProQuest. I just did a search for "Academic / Scholarly" and "Journal" and came up with more than 107,000 different results.