I believe it's just every system within 15 ly. That was the case for Taranis, and there are 11 systems within 15 ly of Leigong (which says it controls 11, even if the map doesn't). There are 25 controlled systems around Indra, but 26 systems within 15 ly, so there's an exception, which makes me think "capped at 25." Every populated system between 15 and 20 ly got an invasion. Meanwhile, every populated system within 10 ly of a control system around Taranis got an alert on its weekly tick, while none of the populated systems more than 10 ly away from a control system around Taranis did, which makes me suspect 10 ly is the range. In which case Indra would hit HIP 22524, Scythia, 70 Tauri, HIP 20485, and Leigong would hit no populated systems. That's just hypothesis at this point, so I'm hoping to confirm/reject next tick, and I haven't found any obvious pattern around the nonpopulated system alerts around Taranis (they aren't particularly spread out, they don't prioritize systems with planets, they don't favor moving towards the nearest Ammonia worlds as far as I can tell).
If it does actually grab all systems within 15 ly on arrival, though, Oya is coming in less than that distance away from Inara and Lhou Mans, which is 7.5 billion population. I'm hoping that systems of that size get a chance to fight back.