How much can I copy from an online article?

There is no "legal percent" that can be used from any kind of a copyrighted work. You will need to get permission from the copyright owner (who not necessarily the creator) of the material and then provide credit to that person or entity. There are some exceptions to copyright protect, usually referred to as "Fair Use." Check with an attorney experienced in copyright issues to see if any of these would apply to your project. BTW: materials created outside of the U.S. are also protected under international treaties.

The ideas are ok to use - the actual articles are copyright protected automatically in the US since 1978. There no set amount that you can use - infringement can be determined with as little as a few sentences. Seek permission - if it is for educational purposes most authors will freely grant it

Your project may qualify as "fair use" of the work of others - but attribution should be given to all source materials. Fair Use Analysis - Transformative uses that repurpose no more of a work than is needed to make the point, or achieve the purpose, are generally fair use. Is the use you want to make of another's work transformative that is, does it add value to and repurpose the work for a new audience and is the amount of material you want to use appropriate to achieve your transformative purpose? Small amounts are generally ok. You can always provide a link to the original online articles - so your students can review the full versions.

There is no "blackline" answer. What you are talking about is "fair use", and this is complicated. Unfortunately, there is no single generalized answer. However the blanket statement "it is legal to use 10% of a book" is incorrect. Depending on the use, you could be liable for copyright infringement. As for ethics, that's a different subject. Ethically, if you use material that came from someone/somewhere else, you should expressly credit the source.

There is no percent for anything. That is a myth.