copied and edited the code from https://github.com/askotos/hotbar/tree/master

hotbar_16/depends.txt

@@ -0,0 +1 @@
+default

hotbar_16/init.lua

@@ -3,9 +3,189 @@ minetest.register_privilege("builder", {
 	give_to_singleplayer = false
 })
 
+local VERSION = "0.1.5a"
+local MODES = {legacy = "legacy", world = "world", session = "session"} -- this redundancy simplifies later checks
+local DEFAULT = {mode = MODES.world, slots = {legacy = 16, world = 10, session = 12}}
+local MOD_STORAGE = {}
+if not core.get_mod_storage then
+  -- MT < 0.4.16
+  MOD_STORAGE.present = false
+  MOD_STORAGE.settings = false
+else
+  -- MT 0.4.16+
+  MOD_STORAGE.present = true
+  MOD_STORAGE.settings = core.get_mod_storage()
+end
+
+
+local stringified_table_keys = function(what, sep)
+  -- what: table
+  -- sep: keys separator (a string)
+  local rc = ""
+  for k, v in pairs(what) do
+    rc = rc .. k .. sep
+  end
+  return string.sub(rc, 1, -#sep - 1)
+end
+
+local new_masked_array = function(mask, max)
+  local rc = {}
+  for i = 1, max do
+    table.insert(rc, string.format(mask, i))
+  end
+  return rc
+end
+
+local read_mode = function(key, default_value)
+  if not core.is_singleplayer() then
+    return MODES.session
+  end
+  local value = core.settings:get(key)
+  if type(value) ~= "string" or #value == 0 then
+    return default_value
+  end
+  value = string.lower(value)
+  if not MODES[value] then
+    value = default_value
+  end
+  return value
+end
+
+local get_mode = function(storage, key, default_value)
+  if not core.is_singleplayer() then
+    return MODES.session
+  end
+  local value = read_mode(key, default_value)
+  local wrong = false
+  if value == MODES.world then
+    if not storage.present then
+      value = MODES.legacy
+      wrong = true
+    end
+  end
+  if wrong then
+     core.settings:set(key, value)
+     core.log("error",
+              "[MOD] hotbar v" .. VERSION ..
+              " automatically changed and saved the mode. " ..
+              "The mode has now been set to " ..
+              string.upper(value) .. ".")
+  end
+  return value
+end
+
+local get_and_set_initial_slots = function(storage, mode_value, key, default_value)
+  local current
+  if not core.is_singleplayer() then
+    mode_value = MODES.session
+    default_value = DEFAULT.slots[mode_value]
+  end
+
+  if mode_value == MODES.legacy then
+    local result = tonumber(core.settings:get(key))
+    current = result or default_value  -- The first time
+    if not result then
+      -- first time
+      core.settings:set(key, current)
+    else
+      current = math.floor(result)
+      if current ~= result then
+        -- result is a float
+        core.settings:set(key, current)
+      end
+    end
+
+  elseif mode_value == MODES.world then
+    local result = core.deserialize(storage.settings:get_string(key))
+    if type(result) == "number" then
+      current = math.floor(result)
+      if current ~= result then
+        -- result is a float
+        storage.settings:set_string(key, core.serialize(current))
+      end
+    else
+      current = default_value -- The first time
+      storage.settings:set_string(key, core.serialize(current))
+    end
+
+  elseif mode_value == MODES.session then
+      current = default_value -- Session initial value
+
+  else
+    current = default_value -- Unplanned case
+    core.log("error",
+             "[MOD] hotbar v" .. VERSION ..
+             ": the specified mode - " .. string.upper(mode_value) ..
+             " - is unmanaged and has been overridden and set to " ..
+             string.upper(default_value) .. ".")
+  end
+
+  return current
+end
+
+local adjust_hotbar = function(name, slots, selected_image, bg_image_getter)
+  local player = core.get_player_by_name(name)
+    player:hud_set_hotbar_itemcount(slots)
+    player:hud_set_hotbar_selected_image(selected_image)
+    player:hud_set_hotbar_image(bg_image_getter(slots))
+    player:hud_set_flags({hotbar = true, wielditem = true})
+end
+
+local hb = {}
+
+hb.adjust = adjust_hotbar
+
+hb.mode = { key = "hotbar_mode" }
+hb.slots = { key = "hotbar_slots", min = 0, max = 23 }
+hb.image = { selected = "hotbar_selected_slot.png", bg = {} }
+
+hb.mode.current = get_mode(MOD_STORAGE, hb.mode.key, DEFAULT.mode)
+hb.slots.current = get_and_set_initial_slots(MOD_STORAGE, hb.mode.current, hb.slots.key, DEFAULT.slots[hb.mode.current])
+hb.image.bg.array = new_masked_array("hotbar_slots_bg_%02i.png", hb.slots.max)
+
+hb.image.bg.get = function(slots)
+  return hb.image.bg.array[tonumber(slots)]
+end
+
+hb.slots.set = function(name, slots)
+  local mask = {err = "[%s] Wrong slots number specified: the %s accepted value is %i.",
+                set = "[%s] Hotbar slots number set to %i."}
+  local display_name = name
+  if minetest.is_singleplayer() then
+    display_name = '_'
+  end
+  
+  slots = math.floor(slots) -- to avoid fractions
+  
+  hb.adjust(name, slots, hb.image.selected, hb.image.bg.get)
+
+  if hb.mode.current == MODES.legacy then
+    core.settings:set(hb.slots.key, slots)
+  elseif hb.mode.current == MODES.world then
+    MOD_STORAGE.settings:set_string(hb.slots.key, core.serialize(slots))
+  elseif hb.mode.current == MODES.session then
+    if core.is_singleplayer() then
+      -- This is an ephemeral / transient storage that is to survive while in a map
+      -- and trying different hotbar modes.
+      -- As a commodity, singleplayer can override the default value to get it back
+      -- if he/she switched back from another mode during the same session.
+      -- This has not to happen on a server to avoid that other players
+      -- overrided it or that their default / current value might be overridden by
+      -- others.
+      DEFAULT.slots[hb.mode.current] = slots
+    end
+  else
+    return
+  end
+  if hb.mode.current ~= MODES.session then
+    hb.slots.current = slots
+  end
+end
+
 minetest.register_on_joinplayer(function(player)
 	if minetest.check_player_privs(player, {builder=true}) then
-		player:hud_set_hotbar_itemcount(tonumber(16))
-    	player:hud_set_hotbar_image("")
+		--player:hud_set_hotbar_itemcount(tonumber(16))
+    	--player:hud_set_hotbar_image("")
+    	hb.adjust(player:get_player_name(), 16, hb.image.selected, hb.image.bg.get)
 	end
 end)