You can connect more than one store to your Selfnamed account. How many depends on the platform.
Shopify and WooCommerce allow one store per platform. For example, you can have one Shopify store and one WooCommerce store connected at the same time, but not two Shopify stores under the same profile.
Squarespace and Wix let you connect several sites from one account. Each site shows up as a separate store in My Stores, with its own products, orders, and billing method.
If you need to run more than one Shopify or WooCommerce store, manage each one under a separate Selfnamed account.
For step-by-step setup, see Can I have multiple Squarespace stores? and the Wix Dropshipping Integration Manual.