In order for a participant to be able to only select one event from a multitude of events, can be achieved in one of two ways.
Option 1 (recommended): OR-relations
Go to 'Module Content' --> 'Registration Events' --> select all the events of which a participant is allowed to only select one --> click 'Create OR Relation(s)'.
The OR Relations column now shows that the choice for each of the events depends on the other events.
On the registration form, after selecting one of the events, the other events cannot be selected anymore.
Option 2: Event Groups
In the R-events overview, click the "Switch to Event Groups" button and "Save".
Click on an event and create an Event Group with a minimum of 0 and a maximum of 1.
Assign all the other R-events to the event group with ID 1. Now all R-events belong to the same group (for which a maximum of 1 R-event applies).
Although this option has the same effect as option 1, it is more time consuming.
Click here for more information on how to setup the event.