Ten thousand people came to her first concert in Chicago, but many less have come to her more recent concerts.
Ten thousand people came to her first concert in Chicago, but many fewer have come to her more recent concerts.
Ten thousand people came to her first concert in Chicago, but many less have come to her more recent concerts.
Ten thousand people came to her first concert in Chicago, but many fewer have come to her more recent concerts.
If we are talking about something we can count, something for which we would ask "how many?" instead of "how much?", then we need to use "fewer."