Rashi on Exodus 19:21/רש"י על שמות י״ט:כ״א

1העד בעם signifies, WARN THEM not to ascend the mountain.

2פן יהרסו lit., LEST THEY BREAK DOWN — warn them that they shall not break down their post because that their longing is אל ה׳ TO THE LORD לראות TO SEE Him and they therefore would approach nearer towards the mountain,

3ונפל ממנו רב AND MANY OF THEM FALL — whatever of them it may be that falls, even though it be a single person only, will be regarded by Me as רב, many (cf. Mekhilta).

4פן יהרסו — The term הרס, “breaking down”, always denotes the separation of the collection of parts that constitute the edifice. Similarly those who depart from the position which people have taken up break down that position.