From Apple’s website:
“The passcodes you use on your Touch ID-enabled iPhone and iPad will now have six digits instead of four. If you use Touch ID, it’s a change you’ll hardly notice. But with one million possible combinations — instead of 10,000 — your passcode will be a lot tougher to crack.”
Craig Federighi, senior vice president of software engineering at Apple, said the new mobile software for iPhones and iPads is focused almost exclusively around proactivity and productivity, without compromising user privacy.
You can read more about all the improvements in the future release of iOS for iPhone and iPad devices at http://www.apple.com/ios/ios9-preview.