The ISSN Portal (https://portal.issn.org) is the single point of access to all ISSN data:
See below the attached PDF!

About the ISSN International Centre for the Registration of Serial Publications
The ISSN International Centre is an intergovernmental organisation which manages the identification and the description of serial publications and ongoing resources, print and online, in all disciplines and at the international level. The ISSN International Centre was officially created in Paris in 1975, under the terms of an agreement signed between UNESCO and France, the host country of the ISSN International Centre. In 2018, the ISSN Network comprises 89 member countries. The ISSN International Centre is responsible for maintaining and publishing the ISSN International Register and for assigning ISSN to publications issued by international organisations, and publications issued in countries which do not have an ISSN National Centre.
(http://www.issn.org)