We've made an improvement in Shopify Markets which now means you can customize your primary market in ways that weren't previously possible.
Shopify preselects the primary market for new stores based on your store's general settings. For example, a store domiciled in Germany with a store currency of Euros will have a primary market of Germany. Previously, you could not change the country or currency of your primary market.
Now, you can change your primary market to a country and currency of your preference, irrespective of your store's general settings. For example, a store domiciled in Canada may want to have a primary market of the United States in US Dollars. You can also customize pricing on your primary market, if you want it to be different than the default values of your store.
The primary market still can only contain one country.
Read more about customizing your primary market in the Shopify Help Center.