If we are to believe Appian -and why shouldn’t we?- Seleucus I Nicator founded no less than nine cities named Seleucia. The most famous of these used to be, probably, Seleucia on the Tigris, according to the cuneiform sources of the early third century the royal residence. Reportedly, it had no less than 600,000 inhabitants, and that may, for once, be an adequate guess (several centuries later, Baghdad had many more inhabitants, but the same resources).
The third Seleucia was founded at the estuary of the river Orontes, not far from Antioch. Its ruins can be visited, which may explain why it is now the most famous of the nine Seleucias. Especially the tunnel, dug by Jewish captives after the fall of Jerusalem, is an amazing sight.