What you need to do is increase the “end frame” in timeline or output properties tab > end frame.
And seconds are relative to your frame-rate. In first image you can see endframe is 250 so i’ll get 250 frames in vse also. and my frame rate is 25 fps so 250/25=10s in vse. In second image is i changed end frame to 500 so now 500/25=20s or 500 frames in vse.