How to draw a set amount of circles at random positions on a screen in pygame?

Its only drawing one circle because thats exactly what your code says. To have multiple circles at random positions you can can generate and store the random position values outside of your loop so it dosent change every frame. Something like this.

sizes = []
noOfCircles = 20
for i in range(noOfCircles):
    size.append([random.randint(0, screenWidth), random.randint(0, screenHeight))

Then in your main loop, you can draw the circle using for loop. Notice in your code, it only draws one circle because you dont use any kind of loop.

for i in range(20):
    pygame.draw.circle(screen,green,(sizes[i][0], sizes[i][1]),radius)

This approach works but i not really the best way to do it. The way its usually done is using classes. So make a class for circle like this.

class Circle:
    def __init__(self):
         #You can initialise the position to a random value
         self.pos = [random.randint(0, screenWidth), random.randint(0, screenHeight)]
         self.color = green
         self.radius = 10

    def draw(self):
         pygame.draw.circle(screen, self.color, (self.size[0], self.size[1]), self.radius)

The you can make a list that hold a bunch of cirlces.

circles = []
for i in range(20):
    circles.append(Circle())

Then in your main loop, you can draw the circles.

while True: for circle in circles: circle.draw()

Here is the complete example code.

import pygame, random, math

win = pygame.display
display = win.set_mode((1200, 600))

class Circle:
    def __init__(self):
         #You can initialise the size to a random value
         self.pos = [random.randint(0, 1200), random.randint(0, 600)]
         self.color = (0,255, 0)
         self.radius = 10

    def draw(self):
         pygame.draw.circle(display, self.color, (self.pos[0], self.pos[1]), self.radius)


circles = []

for i in range(20):
    circles.append(Circle())

def checkIntersection(c1, c2):
    dx = c1.pos[0] - c2.pos[0]
    dy = c1.pos[1] - c2.pos[1]
    d = math.hypot(dx, dy)
    if d < c1.radius + c2.radius:
        return True
    return False


for i in range(19):
    while checkIntersection(circles[i], circles[i + 1]):
        circles[i].pos = random.randint(0, 1200), random.randint(0, 600)

while True:
    display.fill((255, 255, 255))
    pygame.event.get()
    for circle in circles:
        circle.draw()
    win.flip()

Edit: So, to check to intersection, you can just see if any of the circles collide and if they do just generate a new set of random numbers. This insures that thers never intersection. I updated the example code with this implemented.