You don't provide much information about the type of copyright works you wish to use, what sort of course you are intending to run or which jurisdiction is likely to provide the applicable law, so I can't really give you a more detailed answer.
In your other thread you say you wish to use Udemy. That company is based in the USA and operates under US law as far as copyright is concerned. Therefore you may be able to take advantage of the fairly generous fair use provisions for the purposes of education (see Â§ 107
of the US Copyright Act 1976).
However under UK copyright law, although there is no fair use of the sort found in US law, the fair dealing provisions provide a good range of exceptions for educational and general purposes (see Chapter 3
of the Copyright Designs and Patents Act 1988, especially sections 32 to 36A).
And the type of work becomes relevant when it is necessary to judge whether a substantial part of the source work has been quoted. For something like a painting or a photograph, it may be necessary to re-use the complete image for the purpose of instruction, but this then means that a substantial part has been used and this could be grounds for an infringement claim. However where the source is a written work a few sentences from a complete book or article may well fail the substantiality test, and would thus not be infringment. The other thing to consider is why exactly you need/want to use someone else's work. If it is because that work is either the subject of study for the course, or the author is a respected authority on the subject and a quote from them adds academic value to your course, then those are acceptable purposes. On the other hand if you are effectively saving time and effort by using someone else's work then that will generally fail the fair dealing test.
Even without the details, I can tell you that it is impossible to provide a definite quantity (like 20% or 150 words) which is permissible. However before resorting to paraphrasing etc, you should certainly consider seeking permission to quote from the source works.