Zotero is capable of formatting references in a variety of styles. One of the most popular is that of the American Psychological Association (APA), used in psychology and the humanities. According to the 5th edition of the APA Style Manual, "If, and only if, each issue of a journal begins on page 1, give the issue number in parentheses immediately after the volume number" (pp. 227). This means that 95% of the time, the issue number should not appear. So how do you get rid of it?
Well, Zotero has a .csl file for each of its styles, containing code that specifies what information should appear in the reference list and how it should be formatted. To make the issue numbers disappear, you simply remove the code that inserts the issue number into the reference list.
Finding the style file
The style files are located in the Zotero directory, C:\Documents and Settings\YourUserName\Application Data\YourBrowser\Browser\Profiles\xxxxxxxx.default\zotero\styles\. I have written YourUserName because yours will be different to mine, and YourBrowser because Zotero is compatible with any Firefox-based browser, so for most people this will be Firefox, but for others it will be Flock, and so on.
Removing the issue number
Open the apa.csl file and
Update 18/07/2012: As tends to happen when new versions come out, the advice above will not work for newer versions of Zotero. You still open apa.csl with a text editor, but now find and delete the following text:
"text variable="issue" prefix="(" suffix=")"
on line 310.
Here's one we prepared earlier
To make this easy for those of you who aren't tech savvy, I have made my apa.csl file available for download (compatible with Zotero 3.0.8). Just copy it to your Styles folder and overwrite the existing file.