Each Servd project allows you to run a single Craft 'codebase' on the platform. This codebase can be a basic Craft site, or it can be a Craft multi-site.
There's nothing special needed to get a multi-site up and running on Servd, but if your Craft sites are set up to respond to different base domains, you will need to add each of these domains to the Project Settings > Domains page in the Servd dashboard.
Once all of your domains have been added to Servd and you have configured the DNS for each of them appropriately, you will be able to visit your project on Servd using each of them and Craft should differentiate between your different sites automatically.
Note: trial projects are limited to a single domain to prevent loads of them being reserved by a project which isn't actually using them. Once a project is on a paid plan you can add up to 20 domains straight away and you can ask for that limit to be raised at any time.