The ghostIDs are transmitted using 17bits. 2^17 = 131072 ghosted objects at a time. Due to bugs in the ghosting code, if you have more objects than that, you may have problems like not seeing other people or not being able to respawn. Even if it works perfectly, you won't be able to save everything at once from the client because not all of the server's objects will be represented on your client.
I could up the ghost limit but there are problems with that. Say I up it to 19 bits for 524288 ghosts max. That would be an extra 2 bits for every single ghost update packet that is sent out to every client. If you're on the DSL network setting, the maximum packet size is 400 bits and the maximum packet rate is 20. So for a scene with 128k bricks, those extra 2 bits would take 640 packets which would take a minimum of 32 seconds to send out to you. Realistically though, it would probably add over a minute to the brick loading time.
Considering the framerate most people get in the city as it is now, I don't think adding more bricks is a prudent thing to do.
