At the beginning, Stark got only 1 supply. right?

Beginning of The Stark supply is at No.1 supply track [2 armies limit], so they can get a army with 3 unit and more one army with 2 unit.

and they have 2 armies, First one with a footman unit and a knight unit in the Winterfell and second one with a footman unit in White Harbor.

but, why they can have a ship unit in The shivering Sea with that supply limit . *Or i just miss something to know?

You can have unlimited armies of one unit. Single unit armies are not limited by Supply.

thank you so much. I am appreciated that.