int rectCount = 5;

void setup() {
  size(512, 200);
}

void draw() {
  background(255);
  noStroke();
  rectMode(CENTER);
  
  float spacing = width / rectCount;
  float paddingLeft = spacing / 2;
  
  for (int i = 0; i < rectCount; i++) {
    fill(192, 64, 64);
    rect(paddingLeft + spacing*i, height/2, 50, 50);
  }
}