Post what you've done so far so that we can see how you're approaching this.

I don't really have much right now. I've got the grid going, but I had the help with a tutorial.

So basically, inside each of the squares that this code generates, I'm hoping to get a number, preferably beginning at 1, and present it inside the square.
Then i'll be using the squares number as an identifier to detect collisions later on.

int boxsize = 25;
PFont f;
int cols, rows;

void setup() {
  f = loadFont("ArialNarrow-48.vlw");
  size(500, 500);
  cols = width/boxsize;
  rows = height/boxsize;
}

void draw() {
  for (int i = 0; i < cols; i++) {
    for (int j = 0; j < rows; j++) {
      int x = i*boxsize;
      int y = j*boxsize;
      fill(255);
      stroke(0);
      rect(x, y, boxsize, boxsize);
      textFont(f, 10);
      fill(100, 150, 200);
    }
  }
}

1. int boxsize = 25;
2. PFont f;
3. int cols, rows;
4.  
5. void setup() {
6.   size(500, 500); // Always first call in setup()!
7. //  f = loadFont("ArialNarrow-48.vlw");
8.   f = createFont("Arial", 48); // Previous line OK, this one is just more convenient for tests
9.   textFont(f, 10); // No need to call it on each cell, if you don't change it
10.   textAlign(CENTER);
11.   cols = width/boxsize;
12.   rows = height/boxsize;
13. }
14.  
15. void draw() {
16.   for (int i = 0; i < cols; i++) {
17.     for (int j = 0; j < rows; j++) {
18.       int x = i*boxsize;
19.       int y = j*boxsize;
20.       fill(255);
21.       stroke(0);
22.       rect(x, y, boxsize, boxsize);
23.      
24.       fill(100, 150, 200);
25.       text(str(j * cols + i + 1), x + boxsize / 2, y + boxsize / 2);
26.     }
27.   }
28. }

Thanks phi.lho! That's exactly what I needed.

If you don't mind, could I get your help on a second question?

How can I attribute to each of these single rectangles a unique identifier such that rectangle 1 is, for example, R1; rectangle 2 is R2 and so fort? Should I be considering object oriented programming for this issue?

I'm going to need that each individual rectangle detects if it's being 'colided' with by another object i'll be importing into a string.

For such a simple set up, you can just detect the coordinates of the object against the computed coordinates of the rectangles.
For example, replace the fill(255); above with:

1.       if (mouseX >= x && mouseX < x + boxsize &&
2.           mouseY >= y && mouseY < y + boxsize) {
3.         fill(#88FF88);
4.       } else {
5.         fill(255);
6.       }