Multiple people are horseback riding on a track.
Many people are riding horses.
Riding horses is a rephrasing of horseback riding.
exists p1 x1 x2 x3.(on(p1,x3) & n_track(x3) & exists s1 x4.((x1 = x4) & n_riding(x4) & a_horseback(s1) & Theme(s1,x4)) & n_people(x1) & (x1 = x2) & ne_nam_multiple(x2))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_horse(x2) & n_people(x1) & n_quantity(x1))