In my single player game, I've found 2 collectors to meet my needs so far.
In my co-op we'd been using 4, but now that we're pushing into the late game and glue processing has become more important, we've gone up to 6 collectors with mods to keep swimming in that water.
-Edit: Several days later,
I just wanted to amend to this post that in the Co-op world, though 6 collectors was enough for practical purposes, we still had a surplus of bones and we're going to put up a waterfield of another 10 (though we're setting it farther away so that the screamers it would draw wont be a problem.