Q: How many paragraphs do I need to write the background of my research?
There is no universal expectation or convention regarding the number of paragraphs in the Introduction. The purpose of the introduction is generally to explain to the reader the reasons why the study was conducted—the background of previous research, why such a study is needed, any gaps in knowledge or clear directions for subsequent work, and the specific motivations for the research question being addressed in the present study. Thus, the length of the introduction is determined by the amount of information needed to convey these points, limiting this information to what is essential to understand the motivation for the study so that the introduction does not turn into a review of the literature.
With regard to paragraphs, conventionally, the start of a new paragraph denotes a shift in topic or a new thought or idea. Thus, the number of paragraphs in any given piece of text would depend on the number of distinct ideas or concepts being presented, all of which should be linked so that the flow of information from one paragraph to the next is clear to the reader.
Thus, the question that should be asked when writing the introduction is whether the information being presented is essential to help the reader understand the motivation for conducting that specific study, and eliminating any extraneous information that may be interesting but that does not directly serve this purpose.
Hope this helps. All the best for your manuscript!