You could generate a list of users using the following code:
users = gis.users.search(query=None,
sort_field='email',
sort_order='asc',
max_users=700,
outside_org=False,
exclude_system=False)
Then, make the owner in your query dynamic and loop through each user:
for user in users:
# get username
name = user.username
# get items owned by user
itemsList = gis.content.search(query = 'owner: {}'.format(name),max_items
= 10000)
# get number of items
owned_items = len(itemsList)
# print the result (or write to a file, etc.)
print ("{} owns {} items".format(name, owned_items))
(edit: a right parenthesis)