I have an irregular polygon.
I want to make parallel lines with 14 meters of space between the inside of the polygon using QGIS, does not matter the direction, but it is necessary to have the least lines as possible.
Zoom to your polygons extent. Use Processing - QGIS geoalgorithm - Vector Creation Tool - Create Grid to make a line grid at the same processing extent at the current window at your spacing preference. Manual delete the horizontal or vertical lines and save your edits. Use Use Processing - QGIS geoalgorithm - Vector Overlay Tools - Clip to clip the parallel lines to your polygon.