Creating a course registration process in SharePoint

If I understand properly, you have the following business requirements:

• Event which extends the default event with a few more fields such as max number of participants (integer number)
• Registration would take place and add another user to the Participants field (a multi people picker field allowing only persons). When a user is added or removed, the max number of participants would change (you need to ensure that you do not exceed the initially specified number). Specifications are unclear how would a user register - a custom form, leverage on PowerApps - I believe this is your decision based on your knowledge and requirements?
• You would need to ensure that the event is still open for registration - for example, a field for Event State.
• When a list item is updated, a check should happen determining if the number of users has reached 0. Then it would close the event for registration (for example, another field for Event State). This could be achieved with MS Flow or more technically with a web hook - again depends on your knowledge and requirements.

So, I would identify the following technologies based on your development / user knowledge: MS Flow, PowerApps or if you have development knowledge: custom form levereging on SharePoint REST and a web hook hosted in Azure which could leverage on app only.