Out of 31 new born chicks, how many are expected to die in the first few weeks?

we recently got 31 new born chicks, and 6 have died, we have a heat lamp, a fan for when it's hot out, we keep their food and water fresh, we keep the coup clean and we have hay for beding, so we were just wonderin if this many dying in the first couple weeks was normal or if we were doing something wrong.