A little more background from a prosecutor:
We always need way more people than end up actually serving. Let’s say we’re having one felony criminal trial, meaning the desired result is one jury of 12 people. To get that, you’ll probably need to summon about 125 people. Of those, about half will actually show up. The rest will either defer, or not be qualified, or will have moved out of the county and not updated their records, or are just skipping out on jury duty. So, in calling 125, you’re hedging your bets.
So that leaves you with 62. Of those, another 12-15 who show up will either be disqualified, claim an exemption, or have some other excuse as to why they need to be deferred. These can ordinarily be done over the phone, but some people who can be excused show up on the day of jury selection anyway. Frequently, someone shows up who has a felony or theft conviction, and they have to be excused. So, that cuts us down to maybe 50.
Now voir dire begins, and several people end up being challenged for cause and excused. Stuff like, they say they can’t consider the full range of punishment, or they know the defendant’s family, or their immediate family member was raped and they can’t be fair to an accused rapist. That’ll knock out another 7-10. So now we’re down to about 40.
Each side has 10 strikes they can use for whatever reason they want (except a discriminatory reason, like race or religion). If you’re counting, that means that you need to end up with at least 32 (10 strikes + 10 strikes + 12 jurors). There will probably be some double-cuts, but just in case, that’s what you want. Also, in some states (like Texas), alternates are required, so you really need at least 36.
So, calling 125 people for a 12-person jury actually cuts it pretty close. Remember, we’ve been doing this for a while, we know how many people we actually need.
And this would be for one trial in a small county. In large counties, they frequently have multiple trials going at once, so they call hundreds of jurors, qualify them in a jury pool room, and divide out panels into different courtrooms. Also, defendants often decide to accept a plea when they’re actually looking down the barrel of a trial, metaphorically speaking. So, they may call a couple hundred jurors, only to have most of the defendants plead guilty right before trial, and they can send most of the jury panelists home.
I had the reverse happen to me once: I was called in my city, and none of the defendants decided to plead, so they were actually short of jurors. I had to sit through one voir dire in the morning, and stick around with everyone else who wasn’t selected for another one in the afternoon (both were misdemeanor DWI trials, which only require six jurors).
Another thing to consider: potential jurors aren’t the only ones who have to miss work and rearrange their schedules to be at court that day. Victims, witnesses, and their families have to do it too. They didn’t ask to be the victim of a crime. Even though they want to convict the guy who wronged them, coming to court is no fun for them either. And if there aren’t enough jurors and they have to go through it all again, it’s extremely upsetting, especially if you’re a little girl who has to tell a room full of strangers about her first (unwilling) sexual experience. So have a heart.