Is there a charge/cost for updating latest versions of apps: Pages, Numbers, Keynote?

The current (2020-05-24) version 10 of Pages, Numbers, and Keynote require the installation of Mojave 10.14 or Catalina 10.15. With these application versions installed, the Updates panel in the Mac App Store application will indicate any available free updates as they are released. A red badge with count, will appear on the Mac App Store icon in your Dock when any updates are available in the Mac App Store.

There shouldn't be if they are on your Mac now, have you opened Software Update/AppStore?

At top left of screen under the Apple logo, choose AppStore, then click on the Updates icon... anything show up?