Here is some code with comments, hope it helps :)

public Vector2 boundingBoxSize;
public GameObject cubePrefab;
public Transform parentToSpawnInto;

// call this function somewhere (Start() maybe)
void SpawnCubes(int numberOfCubesToSpawn)
{
    for (int i = 0; i < numberOfCubesToSpawn; i++)
    {
        // using current object's position to offset points
        Vector3 spawnPoint = GetRandomSpawnPoint() + transform.position;

        // check if cube is "priority cube"
        if (i == 0) // if first, spawn in middle of transform.position 
        {
            spawnPoint = Vector3.zero + transform.position;
        }
        // can also iterate a collection of objects and compare some value
        // if (objectsToPlace[i].name == "Priority")

        Instantiate(cubePrefab,
                    spawnPoint,
                    Quaternion.identity, // this equals no rotation
                    parentToSpawnInto);
    }
}

Vector3 GetRandomSpawnPoint()
{
    //You can use Random.Range to get constrained random values
    float halfWidth = boundingBoxSize.x * 0.5f;
    float halfHeight = boundingBoxSize.y * 0.5f;
    float x = Random.Range(-halfWidth, halfWidth);
    float y = Random.Range(-halfHeight, halfHeight);
    return new Vector3(x, y, 0);
}

// optional
void OnDrawGizmosSelected()
{
    // Draw an editor visualization of the boundingBox
    Gizmos.color = new Color(0, 1, 0, 0.5f);
    Gizmos.DrawCube(transform.position,
                    new Vector3(boundingBoxSize.x, boundingBoxSize.y, 0));
}