You can have unlimited apps in your account but may only have two apps running concurrently in free tier with 256MB of RAM. You'll be able to process thousands of events every second with these apps. However, these apps need to be re-deployed once every 7 days to identify idle apps running and stopping them in free tier accounts.
When you need to have apps running continuously without having to bother to re-deploy them you can upgrade an app by buying an LCU for $9/month.
When you buy LCUs you automatically migrate to a paid account.
Updated about 2 months ago