What should the last section of a scientific paper be called? Conclusion or conclusions?
I always felt like conclusions were things to take away from a paper, and a conclusion was the end of something (a book).
According to the Little, Brown Compact Handbook, in MLA style the final section is called the "Conclusion", while in APA style the final section will be labeled "Discussion". When using either citation style (and I would assume this holds no matter what citation style you are using), this section will contain your summary/interpretation, the conclusions (plural) that you wish the reader to take away from the paper, and perhaps a call to action or an outline of future work. So you are right,
conclusions [are] things to take away from a paper, and a conclusion [is] the end of something...
though not necessarily the end of a book; a conclusion can be the end of a paper, article, etc.
There is no real right or wrong here. "Conclusion" is quite common; "Conclusions" indicates that there is more than one, although I am sure that is not the intent. It is also possible to replace the conclusion with a "Summary" or "Synthesis" (both often occur in review papers). In some cases the discussion and conclusion is combined in "Discussion and Conclusion(s)". In other cases, the last section might be "Recommendations".
In short, the last section might be called many things but it should summarize the important points of the discussion. That is the main point of "Conclusion" and its "relatives". So from the point of view of content it is all the same.
If only one 'concluding factor', then use Conclusion. If more than one; then use conclusions.