Just because you call it clusterList doesn’t make it a list! It’s basically a lazy iterator, returning each item as it’s needed. You can convert it to a list like this, though:
clusterList = list(clusterList)
Or do that and get its length in one step:
length = len(list(clusterList))
If you don’t want to take up the memory of making it a list, you can do this instead:
length = sum(1 for x in clusterList)
Be aware that the original iterator will be consumed entirely by either converting it to a list or using the sum() formulation.