As many as you like :-)
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But when I add my 'ible to a contest it says it's entered but it's not.
You should get an email saying "entry received", then you'll get another email a day or three later saying it has been accepted (or rejected).
Only worry if you haven't had the "received" email.
Most of the time you can enter one Instructable into three contests and you can enter as many Instructables as you can write.
As long as the Instructyable is published after the starting date of the contest.
If your Instructable is rejected for one contest you can submit it for another.
So it is not quite as many as you like.