The U.S. Code does not explicitly delineate number of words, lines, notes or images that would constitute fair use or not. Consider the following four factors. If more are in favor of fair use, you can use the material in your class. If most factors fall under "less likely to be fair use" you will want to find an alternative, or reduce the amount you share.
The purpose and character of the use:
The nature of the copyrighted work:
The amount and substantiality of the portion used:
The effect of use on the potential market for the copyrighted work:
The TEACH Act addresses digital uses of material for online learning. It provides for the performance and display of copyrighted materials for distance education by accredited, non-profit educational institutions (and some government entities) that meet the Act's requirements.
TEACH requires that the academic institution meet specific requirements for copyright compliance. For the full list of requirements, refer to the United States Code, Title 17, Section 110.
It is also important to note that TEACH does not supersede fair use or existing digital license agreements.