> In the client I use the port number  3050 in the connection string.

That defines the **port on the server** which will be connected to.

The client side port is assigned by the OS, since there is no need to use a specific port on the client side.

Also, it necessary to support a client having multiple database connection to single or multiple servers or databases.

> Is FB use multiple port as the same time ?
> Is there a document which explains that ?

It is basic client side IP port handling.