How do I make a Spawner that selects one random humanoid out of 5 or more different types?

You could use math.random.

Before we begin, let me give you an example of math. random:

local number = math.random(1,3)
print(number)

Output: 1, 2, or 3.

Okay, now to the coding:

First, let's get the number of players.

local Players = game.Players:GetPlayers() -- gettings all players
local nplrs = #Players -- number of players
print(nplrs)

Output: The number of players.

Okay, now let's use the code the get a player's name

local Players = game.Players:GetPlayers() -- gettings all players
local nplrs = #Players -- number of players
local Randomplayer -- random player
     Randomplayer = Players[math.random(1, nplrs)] -- random player
print(RandomPlayer.Name)

Output: A random player's name.

Now, let's get a random humanoid:

local Players = game.Players:GetPlayers() -- gettings all players
local nplrs = #Players -- number of players
local Randomplayer  -- random player
if nplrs >= 5 then -- if more or = to 5 then
    Randomplayer = Players[math.random(1, nplrs)] -- random player
    local randomplayerhumanoid = Randomplayer.Character.Humanoid -- the random humanoid
end

More information: https://developer.roblox.com/en-us/api-reference/lua-docs/math

https://developer.roblox.com/en-us/api-reference/class/Player