How to know if there is no possible space on board to spawn.

I have a method that spawns 4 different type of prefabs in order of priority. it spawns the first type, then spawns the next if there is room. There's a number, of objects per type to be spawned, specified. I want it done in a way that if there's no room for an object of lower priority to spawn, then it doesn't spawn. This is the method I have:

 void Spawn( GameObject obj, float height, int num, int max){
     
     currentObjectsOnBoard = 0;//number of specified objects spawned
     for (int i = 0; i < max; i++) {

         if (num == 0) {
             break;
         }

         float x = Random.Range (startX, (startX + boardW));
         float y = height;
         float z = Random.Range (startZ, (startZ - boardH));
         Vector3 temp = new Vector3 (x, y, z);


         Collider[] hitColliders = Physics.OverlapSphere(temp, spacer);
         int a = 0;
         bool free = true;

         while (a < hitColliders.Length)
         {
             if ((hitColliders [a].CompareTag ("Collectable")) || (hitColliders [a].CompareTag ("Obstacle")) || (hitColliders [a].CompareTag ("Hole")) || (hitColliders [a].CompareTag ("Player"))  || (hitColliders [a].CompareTag ("Coin")))  {
                 free=false;
                 //check if spot is occupied by any of the above, within a radius specified by spacer
             }
             a++;
         }

         if (free) {
             Instantiate (obj, temp, Quaternion.identity);
             currentObjectsOnBoard++;
         } else {
             i--;//i did it like this so the maximum number of spawnable items can be fully reached
         }

         if ((currentObjectsOnBoard == num)) {
             break;
             //if object reaches number specified or if no more can be spawned.
         }
             
     }
 }

How I implemented this was before each object is spawned, I calculate the maximum number of that object that can be spawned after objects of higher priority have been spawned...I think my problem is my math. I think I'm not calculating the maximum number spawnable right. I use the are of each object assuming this was a flat 2d board and I multiply that number by the space I want in between all objects. Here's how I implemented the first two:

 maxCollectablesOnBoard = (int) (((boardArea- (playerArea)) / (collectArea)));
     Spawn (collectable, collectAndCoinHeight, collectNum, maxCollectablesOnBoard);

     tempcollect = currentObjectsOnBoard;//number of collectables spawned if different from numer requested
     maxHolesOnBoard = (int) (((boardArea  - (playerArea) - (tempcollect * collectArea)) / (holeArea)));
     Spawn (hole, holeHeight, holeNum, maxHolesOnBoard);

here's an example of how I calculated the area:

 collectArea = (spacer+1) * (Mathf.PI * collectRad * collectRad);

I've been trying this for days and I can't figure it out, I'd really appreciate the help. Thank you!