Please be mindful when considering material to copy and distribute in your courses. The office will not honor requests that seem to be in violation of copyright law. I hope you agree this sets the wrong example for the student workers who are often tasked with such work. Furthermore, I hope you are not knowingly in violation yourself. From Carla Myers, University Libraries:

In higher education, the fair use exemption found in 17 U.S.C Section 107 is one we can often look to when wishing to make and distribute copies of works to students. When making a fair use determination we must consider:
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.

Working through these four factors…
(1) If the course readings are used as part of course instruction and learning then the purpose of the use can be considered educational, which is good.
(2) If the resource being copied is a textbook then it’s scholarly in nature, which is also favored under the law.
(3) For the third factor, the course instructor needs to carefully consider how much of a work needs to be copied/reused to accomplish their purpose and only use that amount.
(4) For the fourth factor, the instructor needs to consider if the amount of the work they are copying is so much that students should instead be buying the work.

There are no specific rules for the amount that may be copied, although you may have heard rules-of-thumb such as 10% or one chapter. The main determination is whether the amount copied could reasonably substitute for the student purchasing the book/source.