Playing around with original dos lemmings and my new database...
Then I saw that in total there seem to be 666 object in all the 120 levels

Coindidence probably.

Wow. It's hard to imagine anyone bothering to track the total so yeah, probably a coincidence. On the other hand, we know they remove plenty of water objects on the DOS version, so I can conceive the case where someone noticed the post-removal total objects count to be close enough to 666 that they could then tweak some levels to get to that number exactly. But unlikely, yeah.
Yes I was thinking about something like that as well.
And the playing around was just testing the import. It's a firebird sql database.
When testing I just did:
Select count(*) from structure
I feel like they did it on purpose.