Two limits apply, and the smaller one wins.
Your plan has an app limit - the counter at the top right of the Apps tab shows it, for example "2 / 5". When it is full the install form disappears until you remove one.
Your plan also has a memory ceiling, and every app draws on it. This is usually the real limit: five apps that each want a gigabyte will not fit in a two gigabyte plan even if your app limit says five.
Some apps count as more than one. An app that also runs a database or a cache starts several containers, and the card says so - "4 containers", for example. They all share your account's memory.
If you are running out of room, either remove something you are not using or upgrade the plan.