How complete are academic 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.