When should I use “et al.” in IEEE citation format?
In IEEE citation format, you should list the names of up to six authors in a reference on your IEEE reference page. If the source has seven or more authors, just list the first author’s name followed by “et al.” (in italics): “F. Gupta et al., …”
In the main text, if you mention a source with three or more authors, you should use “et al.”: “Fowler et al.  argue that …”
Note that you’re not required to mention author names at all in the text though—just the IEEE in-text citation number is enough, in which case “et al.” isn’t needed: “ argues that …”