To keep track of the score, we'll start by creating a new variable and start it at zero. This is a global variable so make sure to put it outside of the update() and draw() functions.

score = 0

We'll increase it by one everytime we touch the coin...

score += 1

IMPORTANT! We should only increase the score if we touch the coin, so make sure to put the above code indented under if player.colliderect(coin):.

If you try to run this now, it will give you an error. That's because score is a global variable, and we are trying to change it inside the update() function. Whenever we want to change a global variable inside a function, we need to add...

global score the top of the function.

Display the score

To display the score, we will use the screen.draw.text() function.

screen.draw.text('Score: ' + str(score), (15,10), color=(255,255,255), fontsize=30)

The parameters are...

'Score: ' + str(score) : This is the string that we want to draw.

(15,10) : This is the position to draw; x=15 and y=10.

color=(255,255,255) : This is the color of the text, in this case, it is white

fontsize=30 : The size of the font.

Like all the other drawing functions, we'll need to put this inside the draw() function.

At this point...

Right now your code should look like this (new lines are highlighted in yellow)...

import pgzrun
import random

WIDTH = 800
HEIGHT = 600

background = Actor('grass')

player = Actor('p3_front')
player.x = 400
player.y = 300

coin = Actor('coingold')
coin.x = random.randint(0, 800)
coin.y = random.randint(0, 600)

score = 0

def update():
    global score

    if keyboard.up:
        player.y -= 5
    if keyboard.down:
        player.y += 5
    if keyboard.left:
        player.x -= 5
    if keyboard.right:
        player.x += 5

    if player.colliderect(coin):
        coin.x = random.randint(0, 800)
        coin.y = random.randint(0, 600)
        score += 1

def draw():
    screen.draw.text('Score: ' + str(score), (15,10), color=(255,255,255), fontsize=30)

pgzrun.go() # Must be last line

You should now see the score increase everytime you touch the coin.