This is just a general question about adding custom maps in general. The Mapdefinitions file makes reference to the first 32 being reserved for core use, but in my experience nothing beyond the first 5 have ever been used. Does anyone know if adding a map 6 such as line 7 below is even supported by the client? I know that the existing maps can be changed out, Im more interested in knowing if you can add to them?

  RegisterMap(0, 0, 0, 7168, 4096, 4, "Felucca", MapRules.FeluccaRules);
  RegisterMap(1, 1, 1, 7168, 4096, 0, "Trammel", MapRules.FeluccaRules);
  RegisterMap(2, 2, 2, 2304, 1600, 1, "Ilshenar", MapRules.FeluccaRules);
  RegisterMap(3, 3, 3, 2560, 2048, 1, "Malas", MapRules.FeluccaRules);
  RegisterMap(4, 4, 4, 1448, 1448, 1, "Tokuno", MapRules.FeluccaRules);
  RegisterMap(5, 5, 5, 1280, 4096, 1, "TerMur", MapRules.FeluccaRules);
  RegisterMap(6, 6, 6, 7168, 4096, 0, "Sosaria", MapRules.FeluccaRules);
As far as I know, the best way to do it is with the Ultima Live as Goat mentioned. As it's been explained to me, the client can't read beyond those map packets (unless a new map packet is sent or created) and that is why that is limited.
Alright, well thanks again for the quick responses, things are slowly coming back memory wise as to what the limitations are.
I too am on the hunt but with a slight plot twist. I'm running a high seas shard which uses .UOP file formats for its map files. I was entertaining the idea of spinning up a couple custom maps from the past (original mul format) to see if they can be replayed. "Zwischenwelt" is one of the maps. I also want to retain the previous OSI maps.

Is MapDefinitions.cs still limited to just 6 core maps? I'm also struggling with understanding the index value as it makes me believe that putting in #33 or higher is safe but how does that link up to the custom map file name.

Another gotcha I'm attempting to solve for is converting old maps and loading them up in UOFiddler & CED. UOfiddler seems to honor the mul file format only and CED keeps farting out on the map size.