Each template is everything you need to host a deployment on SpatialOS. Each deployment needs one template, with the size of the template dependent upon the complexity of the deployment. For session-based games, this means you would normally need a different template for each match.
If you have a large deployment that requires multiple servers, you still only need a single template for your game world, albeit a larger one. Larger templates contain multiple workers and can spread across multiple servers. For example, the w16_r2000_e20 contains 16 server-side cores for workers, as well as egress and the SpatialOS runtime. You do not need to combine multiple templates yourself for a single large deployment.
You can use multiple templates for your game, but only one template per deployment. Hope this helps.