Don't take the following as an attempt to translate but just as some strokes to depict notions:
por implies the notion of or going by or through.
en implies the notion of in, on or at.
más gente por la calle = más gente transitando/circulando (the street/s look/s busier)
más gente en la calle = más gente reunida/mayor densidad de gente (more people on/in the street; mucha gente en la calle = the street/s look/s kind of packed)
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