docstr: The number of concept drifts detected per ensemble member.
code: def n_drifts(self) -> collections.Counter:
      """The number of concept drifts detected per ensemble member."""
      return self._n_drifts
  
imports: from __future__ import annotations
  import abc
  import collections
  import math
  import random
  import sys
  from river import base, drift, metrics, tree, utils
  from river.tree.nodes.et_nodes import ETLeafAdaptive, ETLeafMean, ETLeafModel
  from river.tree.splitter.random_splitter import RegRandomSplitter
label_desc: Use image node_id 14 for calling the ExtraTrees obj's underlying member method code with example usage: obj.n_drifts() and returns: self