Here’s an example wordpress_sites set up with multiple sites (some config formats out-of-date).
If/when you add a new site to wordpress_sites, you would need to run server.yml just for the letsencrypt role (if you’re using Let’s Encrypt) and the wordpress-setup role. The wordpress-setup creates an nginx conf for your new site. You could add --tags letsencrypt or --tags wordpress to your command to run only these roles. Then you would need to run deploy.sh or deploy.yml to deploy the new site.
Running server.yml or deploy.yml will never destroy any sites on the server.
Explore these threads (and their links) for ideas on multiple sites with one Trellis:
https://discourse.roots.io/t/best-practice-for-multiple-sites-on-multiple-servers/5385
https://discourse.roots.io/t/one-trellis-box-multiple-sites-on-individual-vps/7784
https://discourse.roots.io/t/one-do-droplet-per-trellis-bedrock-project/4712