public class Spawner : MonoBehaviour
{
    public GameObject EnemyPrefab;
    public float respawnTime = 260;

    void Update()
    {
        if (respawnTime <= 0)
        {
            Instantiate(EnemyPrefab);

            respawnTime = 260;
        }
        else
        {
            respawnTime = respawnTime - 1;
        }
    }
}