68 mph in a 50 mph zone attracts a penalty of a disqualification of 7 – 28 days OR 4 – 6 points, as well as a Band B fine.
When you respond to the NIP, it is not an admission of guilt; it is merely you providing the Police with your driver details. You should ensure that you do this within 28 days of receiving the NIP. You will then likely receive a Single Justice Procedure Notice asking you to plead Guilty on the papers; Guilty with a Court appearance; or Not Guilty.
If you wish to avoid penalty points then it may be advisable to plead Guilty with a Court appearance to give you the best possible opportunity of avoiding a ban.