Solo pool: Hardcode limit/num_processes to 1 for correct prefetch count calculations. Closes #2925

celery/concurrency/solo.py

@@ -22,6 +22,7 @@ class TaskPool(BasePool):
     def __init__(self, *args, **kwargs):
         super(TaskPool, self).__init__(*args, **kwargs)
         self.on_apply = apply_target
+        self.limit = 1
 
     def _get_info(self):
         return {'max-concurrency': 1,