The sea level rises. How?
Which sea level are you referring to? There are six in the world presently.
Is there a finite amount of sea? Or is it infinite?
It is a constant for practical purposes.
Where does the extra sea come from? What holds it up?
It is a constant so there is no 'extra'.
Or does the earth get flooded from space at regular intervals?
Not as far as I know.
And how much land was there and was it spread all over the planet or was it concentrated at one particular point, or for that matter at several points?
As I have already pointed out, the land area is constant though which parts happen to be covered by water varies from time to time.