The maximum number of bricks you can have is limited by the largest ghostID that can be transmitted using 17 bits.
2^17 = 131072 maximum possible ghosted objects.
Datablocks and world objects also use those ghost ids, so it varies slightly depending on how many add-ons you have loaded, what map you're on and how many bullets you happen to be shooting at the moment. The 128k soft limit is plays it safe, allowing for maximum datablock usage (2048) plus a thousand or so map and game objects.
Thanks, now I understand a lot more.
