How were harbors built in antiquity and how many ships could they hold?

Answer 1

Most were not "build", but found. They would look for places where Mother Nature had already given them a natural harbor. Bays, coves, etc. And then simply added some piers, etc. to make ship access easier.

How many ships? Depended on the size of the harbor. Big cove, lot of ships. Little cove, few ships.