On Memrise, you can access and learn as many languages as you want at the same time!
To switch between the courses you are currently learning, simply tap the menu icon on the left (on Android), or the course title (on iOS). To start a new course, tap 'Learn another course' at the bottom of that page.
More information on how to switch between courses can be found here.
On the website, you can see all your active courses on your home page.