To increase or decrease the amount of frames that your onion skin reach you have to go to Edit > Preferences > Tools > and then decrease the “Number of previous onion frames shown” to 1. You can do the same for the keyframes after the current frame.
In the future this will be easier to do from the timeline, but for now this is the procedure. Let us know if there’s anything else we can help you with.
@ge0rgetheviking Also I noticed you are using a really old version of Pencil2D. I highly encourage you to use the latest Nightly Build for windows pencil2d-win64-2017-08-06.zip which you can find here for 64 bits: