That's the number of states in which some part of the electoral systems are known to have been hacked by Russians. The real number could very well be higher. Fun snippets:
In Illinois, investigators found evidence that cyber intruders tried to delete or alter voter data.
...
The new details, buttressed by a classified National Security Agency document recently disclosed by the Intercept, show the scope of alleged hacking that federal investigators are scrutinizing as they look into whether Trump campaign officials may have colluded in the efforts. But they also paint a worrisome picture for future elections: The newest portrayal of potentially deep vulnerabilities in the U.S.’s patchwork of voting technologies comes less than a week after former FBI Director James Comey warned Congress that Moscow isn’t done meddling.
Bolds are mine.
Charlie Pierce has something to say about all this:
We are creeping ever closer to actual evidence that there was Russian ratfcking of the vote totals in the last election. Not long ago, people wouldn't even suggest that out loud. We were made vulnerable to something like this because of the interference by the Supreme Court in Bush v. Gore, by the curious goings-on in Ohio in 2004, by a relentless campaign to convince the country of an imaginary epidemic of voter fraud, and by a decade of voter suppression by any means necessary.
What I want to add is this: Let's go back to paper ballots and hand counting. Let's have voting scheduled for two consequent days, such as Saturday and Sunday. Let's make federal rules which stipulate equal access to voting equipment, compared to populations of voters, in all districts. And if the voter ID requirements remain, let's make those voter IDs free and easy to get hold of for all people.
Finally, Pierce surmises that Obama decided to stay mum about all this before the election "so as not to undermine the public's confidence in the integrity of the elections".
Perhaps. But that's a bit like not telling someone about a possible (but not certain) cancer diagnosis so as not to make them upset and worried and afraid, even though it also leaves them in the dark about possible treatments.