Upon initial shipment of the Librem 5 in 2019, it will offer the essentials: phone functionality, email, messaging, voice, camera, browsing. The featureset will expand after shipment and over time to more free software applications. Your user experience will improve as we incrementally add commonly requested applications and features (such as calendaring, notes, calculator, PDF viewer, etc.) while keeping performance in mind.
And since they're building on existing open-source systems like GTK and Flatpak, if you don't want to wait for them to port a calculator to the device, you could presumably port one yourself. The source for GNOME Calculator is right here: https://gitlab.gnome.org/GNOME/gnome-calculator ;-)