Choose system name wisely — it is the address of your app (like yourapp.directual.app), it cannot be changed in the future. But, certainly, you can use your own domain address.
There is an initial limit of 5 apps per user account. If at least one of your apps in on paid plan, the number of apps is not limited.
You can create a blanc app and build it from scratch, or use one of the free templates. That is a great way to get up and running quickly and learn the platform. Some templates require integrations (e.g. with email or Telegram) which can be configured right here.
Click to "settings" icon on app card to edit app details: