The density of crowds differs during low season between the northern and southern areas of the coast. The Humboldt current brings perfect wind to Southern beaches near Lima and Punto Hermosa, so all surfers move there. But due to low temperatures and constant fog "la garua" the coast and Lima don't receive as many crowds of tourists as usual.
Only some days can become really crowded when national holidays are celebrated. In particular, they are The Festival of Sun in Lima, National Pisco Day, or Peruvian Independence Day.