How do I make this pistol reload when the R key is pressed, and also show the amount of ammo?

That's not how mouse.KeyDown works, but I'd recommend using UserInputService anyway, as mouse.KeyDown is deprecated.

Everything is explained in the script:

local tool = script.Parent
local player = game:GetService("Players").LocalPlayer
local ammo = 8
local reloading = false
local mouse = game.Players.LocalPlayer:GetMouse()

function reload(amount) --// Create a reload function to avoid retyping.
    tool.Name = "Reloading"
    reloading = true
    wait(2)
    ammo = amount
    tool.Name = "Pistol " .. tostring(ammo) --// Show the new amount of ammo.
    print("Reloaded")
    reloading = false
end


tool.Equipped:connect(function(mouse)
    print("Pistol Equipped")
    game:GetService('UserInputService').InputBegan:connect(function(input, gameproccessed) --// Get UserInputService, connect a function.
        if input.KeyCode == Enum.KeyCode.R then --// If the key pressed was R,
            reload(8) --// Call the reload function.
        end
    end)
    mouse.Button1Down:connect(function()
        print("Mouse Clicked")
        if (not reloading) and ammo > 0 then
            ammo = ammo - 1
            tool.Name = "Pistol " ..tostring(ammo) --// Show the new amount of ammo.
            local ray = Ray.new(tool.Handle.CFrame.p, (mouse.Hit.p - tool.Handle.CFrame.p).unit * 200)
            local part, position = workspace:FindPartOnRay(ray, player.Character, false, true)

            local beam = Instance.new("Part", workspace)
            beam.BrickColor = BrickColor.new("Gray")
            beam.FormFactor = "Custom"
            beam.Material = "Neon"
            beam.Transparency = 0.75
            beam.Anchored = true
            beam.Locked = true
            beam.CanCollide = false

            local light = Instance.new("PointLight", beam)
            light.Range = 15

            local distance = (tool.Handle.CFrame.p - position).magnitude
            beam.Size = Vector3.new(0.3, 0.3, distance)
            beam.CFrame = CFrame.new(tool.Handle.CFrame.p, position) * CFrame.new(0, 0, - distance / 2)

            game:GetService("Debris"):AddItem(beam, 0.1)

            if part then
                local humanoid = part.Parent:FindFirstChild("Humanoid")
                if not humanoid then
                    humanoid = part.Parent.Parent:FindFirstChild("Humanoid")
                end
                if humanoid then
                    humanoid:TakeDamage(30)
                end
            end
        else
            reload(8) --// Call the reload function.
        end
    end)
end)

Remember, if you don't know how to do something, don't be afraid to look it up on the wiki. Hope this helped.