Mr 22%

Sperm - Sleek, Fast and Focused: The Cells That Make Dad Dad - New York Times

This light-hearted look at sperm cells explains why men have to make so many of them:

...the majority of sperm couldn’t fertilize an ovum if it were plunked down in front of them. “Only a perfectly normal sperm can penetrate an egg,” said Dr. Harry Fisch, a urologist at Columbia University Medical Center, “and the majority of sperm are abnormally shaped.” Some may have pinheads, others have two heads, some lack tails, a third don’t move at all. As a rule, Dr. Fisch said, a man is lucky if 15 percent of his sperm are serviceable. “One guy I saw had 22 percent,” he said, “but that’s rare.”

I didn't realise the figure was so low, even for fertile guys. Still, even with, say, 10% being effective, the article points out that the average ejaculation presents 150 million in total. So that's 15,000,000 good quality, potentially egg breaking, sperm each time.

Still sounds like very inefficient design.