All new ProtonMail accounts come with one email address. You can choose between protonmail.com or protonmail.ch when you create your account.
Once you create your account with .com you can not switch it to .ch,and vice versa.
You can upgrade your account to Premium and own both of the domains. (How to become a Premium account?)
We have made some changes for the new users. When you create a new account, you can choose which domain you want to use, .com or .ch. Once the account is created you can only use the chosen domain. You can use both domains if you upgrade to a premium plan, but your default domain will always be the one you have registered, and if you go back to a free plan, only that domain will be available.
To add additional aliases you will need to upgrade your account.
Please see this article for a comprehensive overview of all your address/alias options: Addresses and Aliases.