6 Onion Skins settings
From the Video Timeline menu select Enable Onion Skins, then go back and select Onion Skin Settings… The Onion Skin Count is the amount of frames you want visible either side of the current frame. The Frame Spacing sets the number of frames between the displayed frames. The Max Opacity percentage is for frames before and after the current frame; Min Opacity is the strength of the outermost sets of frames. The Blend Mode controls how these settings are applied to the preview.