Translation:Just the introduction has twenty pages.
I've never come across "only's" as a contraction of "only is". Actually, although Duolingo's current translation is literally correct, perhaps the meaning is closer to "The introduction alone is twenty pages (long)".
I'm not so sure. My version, if not Duolingo's, can be seen as a comment on the total length of the document, as in "Wow! This a long document, it has a 20 page introduction!". By saying "The introduction itself is 20 pages (long)" you lose that sense. There is also the fact that your version is less literal and is better suited to something like "A própria introdução tem vinte páginas".
"The introduction alone is twenty pages long" should absolutely be accepted.
I asked DL to accept the following: "The introduction has only twenty pages." "is only.. would be "é apenas" or "é só" (is just). Correct me if I'm wrong.
I don't think it has the same meaning. DL sentence shows that the introduction is too long or even that the book/essay is really long! Similar to "The introduction alone..."
Is there any de... phrase I could add at the end? Like, when talking about depth, you say de profundidade, or height, de altura. Is there anything that can be added here?