The 'Lighter' Side Of The Force
As to the actual number, it can only serve as meet suspicion. I seems very likely that sidious had a backup plan. The real question remains: how high of a threat did sidious think vador was and did he ever expect to use order "67". Given the trend of dark side apprentices betraying their masters, I would say sidious was ready to do whatever had to be done.
I think palpatine planned to eliminate vader in return of the jedi. Vader was ordered to his command ship and told to remain there. The emperor believed the rebel attack would fail, after which he could eliminate vader using the death star superlaser... then finish completing the death star with no ressistance