You can just make the tileset image larger, up to a max size of 256x256, which brings you to a max of 256 tiles in one tileset. You'll have to set colission for your new tiles though, but you can do this by right clicking the tiles in BL.
If you want to have any more than that without doing asm stuff, you're gonna have to make use of a trick involving the way tilesets get loaded, but instead of a map with a massive tileset you just get a map with variable tiles based on from what map you entered from.
As for deleting maps, you can just right click the map you want gone in the map listing and press Delete, the files will still exist but the game and BL should just not consider it part of a map.