For example, see the Australian Government sample size calculator at: https://www.abs.gov.au/websitedbs/d3310114.nsf/home/sample+size+calculator; or the United Kingdom Government population proportion, sample size calculator at: https://select-statistics.co.uk/calculators/sample-size-calculator-population-proportion/.
Monitoring and evaluating digital health interventions: a practical guide for conducting research and assessment. Geneva: World Health Organization; 2016 (https://apps.who.int/iris/bitstream/handle/10665/252183/9789241511766-eng.pdf, accessed 16 November 2020).