One, coming face to face with herself 30 years older would put her into shock, and she'd simply pass out, or two...
6 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 00:26:00.098 in the Blu-ray version of the movie.
Quote context
[...]
- You mean I'm gonna see where I live? I'm gonna see myself as an old man?
- No, no, no, Marty. That could result in...
- Oh, Great Scott! Jennifer could conceivably encounter her future self!
- The consequences of that could be disastrous!
- I foresee two possibilities.
- Granted, that's a worst-case scenario.
- The destruction might, in fact, be very localized, limited to merely our own galaxy.
- Let's go. I sure hope we find Jennifer before she finds herself.
- The skyway's jammed. It's gonna take us forever to get there.
[...]