Why can't this script locate multiple enemies at a time?

Your enemyDetected only returns the first enemy it finds, not every single enemy visible!

To fix this, I made it return a Table of all matches, rather than the first match:

local _FindCharacter = script.Parent.Parent.Parent.Parent.Parent.Character
function ClearMiniMap()
    for i,v in pairs(script.Parent:GetChildren()) do
        if v:IsA("Frame") and v.Name == "Enemy" or v.Name == "Neutral" or v.Name == "Friendly" then
            v:Destroy()
        end
    end 
end

function enemyDetected(pos)
	local detected = {}

	for _, xlist in ipairs(workspace:GetChildren())
		if xlist:IsA("Model") and xlist ~= script.Parent then --Directly accessing the className is not recommended.
			local islarm = xlist:FindFirstChild("Left Arm")
			local ishuman = xlist:FindFirstChild("Enemy")
			local torso = xlist:FindFirstChild("Torso")
			if torso and islarm and ishuman then --`~= nil` is never necessary.
				If (torso.Position - pos).magnitude < 50 then --Models don't have a Position property!
					table.insert(detected, torso)  
				end
			end
		end
	end 

	return detected
end

while wait(4) do
    ClearMiniMap()
    if _FindCharacter ~= nil then
		local targets = enemyDetected(_FindCharacter.Torso)
		for _, target in ipairs(targets) do
	        if target ~= nil then
	            if target.Detected == true then
	                local CopyEnemy = script.Parent.Storage.Enemy:Clone()
	                CopyEnemy.Parent = script.Parent
			
			--I assume this is a GUI element?
	                CopyEnemy.Position = UDim2.new(0, (_FindCharacter.Torso.Position.x - target.Position.x),
				0, (_FindCharacter.Torso.Position.z - target.Position.z))
	            end
	        end
		end
    end
end