Script for spawning a number of prefabs in a grid formation in Unity. Place it on an GameObject, right click on the component in the inspector and select "Spawn Elements" to run the spawn method

ObjectSpawner3DGrid.cs

using UnityEngine;

namespace FMPUtils
{
    public class ObjectSpawner3DGrid : MonoBehaviour
    {
        [SerializeField] GameObject spawnPrefab;
        [SerializeField] Transform spawnParent;
        [SerializeField] Vector3 startPosition;
        [SerializeField] Vector3 xUnit = new Vector3(1f, 0f, 0f);
        [SerializeField] Vector3 yUnit = new Vector3(0f, 0f, 1f);
        [SerializeField] Vector3 zUnit = new Vector3(0f, 1f, 0f);
        [SerializeField] int elementsToSpawn;
        [SerializeField] Vector3Int cellSize;

        [ContextMenu("Spawn Elements")]
        private void SpawnElements()
        {
            int rowWidth = cellSize.x;
            int rowHeight = cellSize.y;
            int widthXHeight = rowWidth * rowHeight;
            for (int i = 0; i < elementsToSpawn; i++)
            {
                int z = i / widthXHeight;
                int xySubIndex = i % widthXHeight;
                int y = xySubIndex / rowWidth;
                int x = xySubIndex % rowWidth;
                Vector3 spawnPos = startPosition + z * zUnit + y * yUnit + x * xUnit;
                var objectInstance = Instantiate(spawnPrefab, spawnPos, Quaternion.identity, spawnParent);
#if UNITY_EDITOR
                UnityEditor.Undo.RegisterCreatedObjectUndo(objectInstance, $"Spawn grid element at index {x}, {y}, {z} and position {spawnPos}");
#endif
            }
        }
    }
}