Why does the number 666 occur 66 times in the "Hymn to Wikipedia" page on IMSLP?

I'm not sure I understand how you're counting the instances (my browser comes up with 63, but I see more than 70), but I think the high incidence of "666" is on purpose.

On the IMSLP, files are assigned a sequential 5-digit number at the time they're uploaded. The very first file uploaded to the website would be #00001, the second #00002, etc.

Look at the upload dates: "Picardy Third" uploaded dozens of files on June 8, because the sequential numbers being handed out that day were #666xx. And bass clarinet 2 was assigned #66666.

But I'm not sure what it means. Note that one of the IMSLP users to edit that page goes by the screen name "Perlnerd666."

It is a probably a reference to the number of the beast.

However this is an incorrect reference as 666 is modern myth, the original number of the beast was 333