Photo is a little blurry but here is my setup with 4 headsets on 2 base stations. You can just see one of them in the top right corner of the photo almost 3metres high on the wall. The other is on a carpeted partition wall on the left just out of frame around 2.4m high.
I am using an epic 15m sync cable between the lighthouses and it works fine. The only trouble is I have the lighthouses slightly too far apart at approx 8m. Their effective range is 5.5m which means that the two VR stations on each end of the line are effectively running on one lighthouse. Having said that, if I turn one lighthouse off and on again I can clearly see an improvement in tracking on the end stations using both, so it is still helping at that distance.
With the players all in a line I do get a bit of occlusion though, especially for tall people when they bend down to try and pick something up in VR. Kids have it much easier for this. I haven't had any major complaints, but eventually I'll be looking to go with Vive Pro or Pimax for the 2.0 lighthouses to eliminate those dead zones.
I think the limit (according to the FCC) is 79 channels for the controllers, which would mean 39 Vives is the theoretical maximum (unless you're using different controllers such as flight sticks or racing wheels I guess).
