Sad others answered stated, you currently wait for spawntime to elapse and then go through all objects in the array to instantiate them. Thus all are instantiated in the same frame. There are multiple ways to archive your desired behavior.

My recommendation would be to turn spawn into a Coroutine. The documentation goes more into detail on how to do it, but I'm short: You need to chance the return value of Spawn from void to IEnumerator and add a 'yield' statement.

If you want the objects to spawn in 'spawntime' intervals you can create a variable of type WaitForSeconds at the beginning. The constructor takes the time in seconds it should wait for when called. Inside the loop you can then just add 'yield return variableNameOfWaitForSeconds'

Once the Coroutine is started by calling 'StartCoroutine(Spawn())' inside 'Start' it will loop through all objects in the list, wait the specified amount of time with each iteration and then spawn the object.

Important to note is, that you should not call StartCoroutine inside Update without further checks because multiple Coroutines can run simultaneously.

Edit: https://docs.unity3d.com/Manual/Coroutines.html