it seems to be capped at 6 towers, but have a look:

I also built 6 towers at the top (so 420 red for 12 towers) and when i got a unit over there i saw almost the whole map.
Why is every aspect of this synapse exclusive but not the sight radius btw?
- edit -
did some more testing that might represent an actual problem in the current game:
http://i.imgur.com/NJV2g1W.jpg




