I am working through creating webhook subscriptions in sandbox - and I created the 3rd one and went to the list in developer dashboard to make sure it got added correctly.
When I refreshed to see - there was a message about Endpoint Limit Reached, and to contact support if needed more.
First, is this a sandbox only limit?
Second, if not just sandbox, is it limit on subscriptions, or limit on what different notification URL’s can use?
The limit is for both sandbox and production and the limit is only on the notification URL’s. You can subscribe to multiple or all events for a single URL. 
@Bryan-Square
ok thanks - I can make this work - I was planning on grouping all related event types into subscriptions. Such as all order related events into one subscription.
Though one followup question - I was using a webhook.site url for testing - was the limit hit because I used that domain 3 times? If I had different notification URL paths from our actual domain would it allow more than 3 subscriptions?
Great! The limit was hit due to the 3 independent subscriptions. The URL itself didn’t trigger the limit. The default limit for an application is 3.