I haven't tested enough to make any conclusive claims, but here's what I believe:
Map Pieces with a PRT file numbered over 1024 will show up, can be assembled, and saved (aka recorded in a .MAP file) in the map editor.
However, the editor will crash when you try to 'output' (aka convert the .MAP file to an .MPX file) a map with a PRT # over 1024.
MPX files are what SoM actually renders in-game maps from. An MPX file contains a copy of the MSM and MHM files (aka the mesh and collision shape) for every Piece used on the associated map and they can only contain 1000~ distinct map Piece models. However, each of those 1000~ map Piece models can be 3x3 in size.
Each map has its own distinct MPX file (with the map Pieces embedded in it) so you should be able to use up to 1024 unique Pieces on each map. For example, using the 'multi-install' setup, you could open a project in a SoM install that has 1024 custom made 'water-world' map Pieces and use all of them to build Map #1 and save/output Map#1. Then, open the same project in a different install with 1024 custom made 'jungle-world' map Pieces. Use the jungle Pieces to build map #2 and save/output map #2- leaving the MPX file for map #1 unchanged. You should then be able to play through both maps (which contain a total of 2048 different Pieces) without trouble in the final game.
If you made a custom program to convert MAP files to MPX files that didn't automatically crash if the Piece's PRT file was numbered above 1024, I think you could have thousands of map pieces available in the editor as long as you didn't use more than 1024 on any one map.
Map Pieces with a PRT file numbered over 1024 will show up, can be assembled, and saved (aka recorded in a .MAP file) in the map editor.
However, the editor will crash when you try to 'output' (aka convert the .MAP file to an .MPX file) a map with a PRT # over 1024.
MPX files are what SoM actually renders in-game maps from. An MPX file contains a copy of the MSM and MHM files (aka the mesh and collision shape) for every Piece used on the associated map and they can only contain 1000~ distinct map Piece models. However, each of those 1000~ map Piece models can be 3x3 in size.
Each map has its own distinct MPX file (with the map Pieces embedded in it) so you should be able to use up to 1024 unique Pieces on each map. For example, using the 'multi-install' setup, you could open a project in a SoM install that has 1024 custom made 'water-world' map Pieces and use all of them to build Map #1 and save/output Map#1. Then, open the same project in a different install with 1024 custom made 'jungle-world' map Pieces. Use the jungle Pieces to build map #2 and save/output map #2- leaving the MPX file for map #1 unchanged. You should then be able to play through both maps (which contain a total of 2048 different Pieces) without trouble in the final game.
If you made a custom program to convert MAP files to MPX files that didn't automatically crash if the Piece's PRT file was numbered above 1024, I think you could have thousands of map pieces available in the editor as long as you didn't use more than 1024 on any one map.