What does "do the loneliest number" mean there?
Context:
One is the loneliest number that you’ll ever do, unless, of course, you’re a hermaphroditic worm.
The free-living flatworm Macrostomum hystrix prefers to mate with others of its species; but if placed in isolation, the independent worm will do the nasty on its own for the sake of reproduction. Unfortunately, there’s still a price to pay for being a loner: In order to self-inseminate, worms have to inject sperm into their own heads.
It’s a gruesome reproductive technique that scientists call “cephalo-traumatic secretion transfer via a hypodermic penile appendage.” How romantic.
More:
http://blogs.discovermagazine.com/d-brief/2015/09/18/flatworms-stab-heads/#.VfxB2yXvNZA