According to the German website for Super Mario Party, the game is set to include just four game boards. Those boards are Whomp's Domino Ruins, King Bob-omb's Powder-keg Mine, Megafruit Paradise, and a mystery board, which we believe to be Golden Kingdom. The website specifically says 4 game boards, so it seems that will indeed be the total. I'm sure people were looking for more than that, but hopefully these boards are quite expansive.
If you want to see official footage of the first three boards named, you can check out the clips below.