count = modelname.all(keys_only=True).count()

count = modelname.all(keys_only=True).count(some_upper_bound_suitable_for_you)

count = modelname.all(keys_only=True).count(some_upper_limit)

class GetCount(webapp.RequestHandler):
    def get(self):
        query = modelname.all(keys_only=True)

        i = 0
        while True:
            result = query.fetch(1000)
            i = i + len(result)
            if len(result) < 1000:
                break
            cursor = query.cursor()
            query.with_cursor(cursor)

        self.response.out.write('<p>Count: '+str(i)+'</p>')