* 18F and USDS (USA): https://18f.gsa.gov/ and https://www.usds.gov/
* GSA Technology Transformation Service: https://www.gsa.gov/about-us/organization/federal-acquisitio...
* US Web Design System: https://v2.designsystem.digital.gov/
* Government Digital Service (UK): https://www.gov.uk/government/organisations/government-digit...
* GOV.UK Design System: https://design-system.service.gov.uk/
An - admittedly cursory - search for non-Anglophone governments that promote the use of a design system only brought up the Swiss federal government: https://swiss.github.io/styleguide/en/
Both Finnish and Estonian authorities are said to be working on a styleguide but apart from a guide on the Estonian part that mainly focuses on Estonia as a brand there doesn't seem to be anything publicly available yet: https://brand.estonia.ee/design/
Still, for instance I wasn't able to find anything for either the French or the German authorities in their respective local languages.
For Dutch government services on the other hand there's the Rijkshuisstijl styleguide: https://www.rijkshuisstijl.nl/
(I just didn't find it at first because I searched for something along the lines of "nederlands overheid" rather than the official term "rijksoverheid").
UN guidelines to ensure said infrastructure is available and accessible to a countries population would make sense though.
Sure, there is duplication of work, redundancy etc. But what you end up with is a much wider surface area to try different things, better things, copy ideas and combine good ideas in different ways.
On the contrary, if every country just pools ideas and selects the 'best' via a giant committee selection process, would you really expect that to produce a better result over the longer term? (Genuine question and I'm open to the answer being yes but I personally suspect not).
Given that these are presumably research-based guidelines specific to that government's network of sites, that number would be 1.