Today is Yom Hashoah, Israel's day of commemoration of the Holocaust. From the perspectives of the archives, this is the day to be visiting the online efforts of Yad Vashem. Foremost among them, in its extent and significance, is the Central Database of Shoah Victims, a work in progress in which literally thousands of hours of labor have been invested. The database contains millions of searchable documents: Nazi deportation lists, post-war Soviet lists of victims, Pages of Testimony submitted to Yad Vashem by survivors since the 1950s, and many other types of records. It's a multi-lingual database, which recognizes all the data in a variety of languages, so that a search in English will identify and present results irrespective of the language of the documents themselves.
Some of the victims appear on more than one document; the database is pretty good at finding them even if they are recorded in different languages or even different names (Dora and Dvora, say). This ability, however, makes it hard to know how many specific individuals are in there. The cautious assumption at Yad Vashem is that they have records about more than four of the six million who were murdered; they also assume, however, that perhaps a million of Europe's Jews in the Shoah left no record, no surviving family... no trace.
If you've never used the database, and have no family names to seek, it's still possible to gain an impression of the enormity of the Shoah simply by searching for some small township in a remote corner of Europe. There are 837 records for the tiny town of Sapanta, for example, which you've never even heard of: but the Nazis had, and they made sure to murder all its Jews.