An integer value (default: 500) representing the sample size.
References
Buitinck, L., Louppe, G., Blondel, M., Pedregosa, F., Mueller, A.,
Grisel, O., ... & Varoquaux, G. (2013).
API design for machine learning software: experiences from the scikit-learn
project.
arXiv preprintarXiv:1309.0238.