There isn't any hard caps on how many buildings you can place so you've probably run into a bug. Thanks for reporting it!
Do the "ghost guns" still shoot or do they phase from existence alltogether when you zoom out?
Cheers :)
This happens during "building phase".
I have the camera on the right area where I place turrets, then I move baclk towards the left side and suddenly see gaps in the placement where I knew that I had already placed guns.
So I place guns once again.
Just annoying that I have to double check areas where I had already been