The open source Mattermost project offers self-hosted workplace messaging for teams, where teams own and control their own data.
If the intention is to create a proprietary SaaS service, that is not in scope for the project. Please go to mattermost.org and click on "Why we made Mattermost" for context.
If the intention is to use a SaaS configuration to support one extraordinarily large self-hosted deployment (e.g. public sector), you can contact the enterprise team to discuss a horizontal scaling solution using the Enterprise Edition.