"Is there a recommended sample size or number of students that should be surveyed?"
Our rule-of-thumb recommendation about sample size is that you should plan to have at least 75% of all students complete the survey. Of course, sample selection is also a consideration. You'll want to ensure that a broad spectrum of students (e.g., different grades, sex*, cultural backgrounds, academic performance, etc) are invited to participate.
Many schools like to use the anonymous OurSCHOOL survey as a way of gauging the opinions of students who are disengaged, or at risk, and are unlikely to express themselves in a different forum.
* For surveys, the term "sex" stands for the biological sex of the person at birth, not the "gender" that the person may identify themselves with.