Based on the description you provided, it is possible that you witnessed a satellite train or a constellation of satellites. Several companies and organizations have launched constellations of satellites into low Earth orbit, which can appear as a line of lights moving across the night sky.
One notable example is SpaceX's Starlink satellite constellation, which aims to provide global broadband coverage. These satellites are launched in batches and are known to create visible satellite trains shortly after deployment. They appear as a line of closely spaced lights moving in a straight line across the sky. The small gap you observed between the first 15-ish lights and the trailing three lights could indicate a slight separation between two groups of satellites.
Satellite trains or constellations are becoming more common, so it is increasingly likely to witness such phenomena. However, it is always a good idea to rule out other possibilities such as aircraft, drones, or other celestial events before concluding that what you observed was a satellite train.