As a Tetris player, it's 2 T-pieces and a line-piece. Each Tetris piece is 4 squares (Tetris = tetra (meaning 4) + Tennis). 3 Tetris pieces times 4 squares each = 12 tiles
But isn’t that how many imaginary tiles there are? Thats not counting the tiles that aren’t there. You’re just deciding the number of imaginary tiles is the same as the missing tiles.