Working in CI in a company that has couple dozens of Python packages: I'd say about once a week. There are some weeks with no incidents, and there are weeks when everything is broken for many days straight.
NB. The latest incident was Friday when I've discovered that some CI pipeline ran `setup.py install` that down the lane invoked easy_install, which doesn't have a policy of ignoring bizarre versions s.a. X.Y.Zrc1 or X.Y.Zb2 etc. It ran aground when it was trying to install scikit-learn which wanted NumPy >=X.Y.Z, but it already installed X.Y.Zb1, and it didn't realize that this version should be OK (also, it shouldn't have installed non-release versions anyways).
NB. The latest incident was Friday when I've discovered that some CI pipeline ran `setup.py install` that down the lane invoked easy_install, which doesn't have a policy of ignoring bizarre versions s.a. X.Y.Zrc1 or X.Y.Zb2 etc. It ran aground when it was trying to install scikit-learn which wanted NumPy >=X.Y.Z, but it already installed X.Y.Zb1, and it didn't realize that this version should be OK (also, it shouldn't have installed non-release versions anyways).