Do ball bearings require more than 1 save when moving across the tiles?

Ball bearings cover a square area that is 10 feet on a side. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. But is this for every square they move across?

And if it is once per ball bearings, what happens if they enter a 2nd patch of ball bearings that is connected to the 1st?