When working with Maps, a scale is shown at the bottom left corner of the visual (purple highlight below). The scale represents the ratio between distance on the map and the corresponding distance on the ground. The scale is dynamically updated as you zoom in and out of the map. This scale can be shown or hidden as needed.
Add Zoom Control Buttons
Open the Component ribbon and toggle the Scale Control button to show or hide the scale.
The scale control can also be enabled or disabled from the Settings tab of the Formatting panel:
When enabled, the scale will be added to the bottom left corner of the map visualization (red arrow below):