Don't pick a supervisor who has too many students already. Take a course or courses taught by the supervisor before beginning your research to find out methodological biases. Choose someone you feel comfortable with. Don't hook up with someone who is known to be difficult. It' s not worth the effort.
Many departments assign a pro-tem advisor to students at the beginning of the program.
In some cases, the pro-tem will continue to advise students for an additional year, or until students choose their research supervisors. The pro-tem advisor may or may not become the research supervisor. Conflict should not occur while changing from a pro-tem advisor to a research supervisor. However, incoming students and pro-tem advisors must be absolutely clear from the beginning about their expectations for continuing to work together. Sometimes students may choose to change research supervisors.