Python, Iterate through a list sending batches of 100 records at a time to an API, then appending results to another list
You should just be able to append the returned API list directly to return_list:

dict_list = [This is the list with 9000 dicts]
batch_list = []
return_list = []

for i in dictlist:
    batch_list.append(i)
    if len(batch_list)  == 100:
        return_list.append(API_CALL_FUNCTION(batch_list))
        batch_list.clear()

if batch_list:
    return_list.append(API_CALL_FUNCTION(batch_list))


and your else clause is un-needed.

You should also explore slicing the dictlist instead of iterating through each one.
You can call dictlist[0:100] and it will return a list containing the first 100 elements. dictlist[100:200] will return the next chunck, etc.

Hope this helped! Good luck.