That's not six desktops, that's wallpaper. Because it's not the right size, it's divided into six parts.

Solution 1: Replace the wallpaper with a suitable size as the desktop.

Method 2: Right-click the blank space of the desktop, select Properties, click the second desktop, click your wallpaper, and then select.

Tile on the right, or stretch, and find one that suits you. Look at the picture.