There is no limit to the amount of subpacks you can add. Make sure that the "folder_name": "subpack-folder-name" (not the display name) value (in this case, "subpack-folder-name") matches the folder name in /rp-or-bp/subpacks. The value set to"name": "subpack display name" (display name) is simply the display name for the subpack that shows up in game. For more info on manifest.json, click here.