"I" is singular.
Usually, you would say "I was". But sometimes you use the past subjunctive of "to be", which is "were" in all persons.
This is most common in type 2 conditionals:
1. If I were you, I'd get a haircut.
2. If I were to say you needed a haircut, would you be very offended?
Also after "I wish" and "as if"/"as though":
3. I wish I were in Madagascar.
4. It's as if I were slowly floating up into the sky.