As a 20-year old rookie in 1998, Kerry Wood did the unthinkable. On May 6, he struck out 20 Houston Astros in a complete game, one-hit shutout.
On May 8, 2001, Randy Johnson struck out 20 Cincinnati Reds in nine innings of a 13-inning tilt.
Since then, no one has struck out 20 or more batters in a single outing. Tom Cheney of the Washington Senators once struck out 21 batters in a 16-inning game back in 1962 (he had struck out 13 through nine innings).
To date, Roger Clemens (twice), Wood, Johnson and Cheney are the only pitchers to ever strikeout 20 or more batters in one game. And it may very well stay that way.
After witnessing what happened to Wood's arm after his remarkable rookie season, managers are very hesitant to let their starters throw the many pitches it would likely take to strikeout 20 batters.
Though Wood won the National League Rookie of the Year in 1998, his season came to an end in August due to an elbow injury which caused him to have ligament replacement surgery the next spring.
His career continued in May of 2000, but has never been the same since that fateful season.
In all honesty, that single performance was not the culprit for Wood's elbow troubles. The fact is that he threw 120+ pitches in each of eight different games in 1998, including 133 in his final start of that season.
However, most administrative personnel around baseball look at that 20-strikeout game as the start that ended it all.
He only threw 122 pitches in that game, which by today's standards is considered a lot, but not an outrageous amount (Ubaldo Jimenez recently threw 128 in his no-hitter against the Braves.)
However, the likelihood of someone being able to throw that many pitches and be so dominant as to strike out 20 or more hitters seems to be decreasing.
It may indeed happen again, but to date, it's been almost a decade since the last time someone accomplished this feat.