When conducting user research there is an optimal number of users needed based on the type of research, but the knowledge is scattered across the web. NN/g holds much of the information on qualitative and quantitative studies, while SurveyMonkey has an excellent survey sample size calculator. Here is an infographic to make much of this knowledge readily available in a visual format.

Nielsen, Jakob, and Landauer, Thomas K.: "A mathematical model of the finding of usability problems," Proceedings of ACM INTERCHI'93 Conference (Amsterdam, The Netherlands, 24-29 April 1993), pp. 206-213.

Tullis, Tom, and Wood, Larry. (2004) How Many Users Are Enough for a Card-Sorting Study?, Usability Professionals Association (UPA) 2004 Conference, Minneapolis, MN, June 7–11, 2004.

Nielsen, Jakob. “Quantitative Studies: How Many Users to Test?” Nielsen Norman Group, 26 June 2006, www.nngroup.com/articles/quantitative-studies-how-many-users/.

Guest, Greg, et al. “How Many Focus Groups Are Enough? Building an Evidence Base for Nonprobability Sample Sizes.” Sage Journals, vol. 29, no. 1, 28 Apr. 2016, pp. 3–22., doi:http://journals.sagepub.com/doi/10.1177/1525822X16639015.

Nielsen, Jakob, and Kara Pernice. How to Conduct Eyetracking Studies. Nielsen Norman Group, 2009, How to Conduct Eyetracking Studies, www.nngroup.com/reports/how-to-conduct-eyetracking-studies/.

“Sample Size Calculator: Understanding Sample Sizes.” SurveyMonkey, www.surveymonkey.com/mp/sample-size-calculator/.