How do I choose which countries customers can order from?

Yes. Add the countries you sell to under Settings → Store Settings → Selling Location.

1. Click + Add Country, pick the country (its code and dial code fill in by themselves), leave Status on Enabled and click Add Country.
2. Repeat for each country. Set a row to Paused to hide it for a while without deleting it.

What it changes:

• The Country dropdown at checkout and in the customer's saved addresses only lists your enabled selling countries.
• Manage Tax only lets you add rates for these countries (plus your Store Location country).

Two things to know: if you haven't added any selling country, shoppers see the full country list. And this works by limiting the dropdown; the order isn't checked against the list again when it's placed. To block specific areas, use the Delivery Restrictions plugin.