A simplified version to make things easier. What we are going to do is make the ball and array of balls. this first example is where we are starting from.

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float ballx = 0;
float bally = 200;
float speedx = 2.7;
float speedy = 1.2;
void setup(){
size(600, 400);
}
void draw (){
background (50);
smooth();
ballx = ballx + speedx;
bally = bally + speedy;
if ((ballx > width) || (ballx < 0)) {
speedx = speedx * -1;
}
if ((bally > height) || (bally < 0)) {
speedy = speedy * -1;
}
ellipse (ballx, bally, 10, 10);
}

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Here's the code with 200 balls.

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int ballnumber = 200;
float [] ballx = new float [ballnumber];
float [] bally = new float[ballnumber];
float [] speedx = new float[ballnumber];
float [] speedy = new float[ballnumber];
void setup(){
size(600, 400);
for (int i=0; i< ballnumber; i++){
ballx [i] = width/2;
bally [i] = height/2;
speedx [i] = random (-3, 3);
speedy [i] = random (-3, 3);
}
}
void draw (){
background (50);
smooth();
for (int i=0; i< ballnumber; i++){
ballx [i] = ballx [i] + speedx [i];
bally [i] = bally [i] + speedy [i];
if ((ballx [i] > width) || (ballx [i] < 0)) {
speedx [i] = speedx [i] * -1;
}
if ((bally [i] > height) || (bally [i] < 0)) {
speedy [i] = speedy [i] * -1;
}
ellipse (ballx [i], bally [i], 10, 10);
}
}