How much text should teleprompter glasses show at once?
Show enough text to prepare the next thought while keeping the current position obvious. There is no useful universal line count because display size, font, optical fit and the speaker's reading distance all affect what feels comfortable. Short phrases are a sensible starting point.
Break text where a speaker would pause
A line break between ideas is easier to follow than one imposed halfway through a name or phrase. Keep an important number with its unit and avoid filling the display simply because more characters fit. The next cue should be visible without competing equally with the words being spoken now.
Rehearse with a paragraph containing a name, a number and a deliberate pause. If finding the next line interrupts delivery, shorten the view or improve the current-line emphasis before adding more text.