The leak is looking more false by the day. We've been to star clusters (voyager cluster) and they rarely have more than a handful of planets. Unless Bioware is really planning to shrink the scale of the next Mass Effect to take place in a single cluster with less planets than we have fingers... it sounds more like DLC scale than a game scale. Sounds like whoever created the leak used poor terminology.
Star clusters contain between hundreds and thousands of stars, most of which are likely to have multiple orbiting planets.
The Normandy was barely scratching the surface of each cluster it visited, since it only visited between maybe one and six star systems per cluster. The Voyager cluster would have hundreds of star systems that the Normandy didn't visit.
Whether the Milky Way or Andromeda, you could set an entire game in a single cluster and never run out of places to visit.