There is a new interview on the LA Times with the Harry Potter visual effects supervisor, Tim Alexander!

So how many Inferi lie in the lake?
A couple million? Above water, you’d probably see about a hundred at a time. But when Harry gets dragged into the lake, there is a whole underwater environment…and it’s actually covered in bodies. It’s all just … bodies crawling on top of each other, and that’s how you get into the millions.

That sounds … disturbing. Certainly, more so than the previous “Potter” films.
It’s certainly much bolder and scarier than we imagined that they’d ever go in a “Potter” movie. Director David Yates was really cautious of not making this into a zombie movie, so we were constantly trying to figure out how not to make these dead people coming up look like zombies. A lot of it came down to their movement – they don’t move fast, but they don’t move really slow or groan and moan. We ended up going with a very realistic style. They move like anyone coming up out of water.