I would like to issue ClientID & ClientSecret for 3rd parties like this case.
The cap on the number of applications is 100 for the Developer Pro plan, but I think that cap is too low for anyone to be able to issue ClientIDs (like the Facebook API).
In this case, do I have to sign up for the Enterprise plan?
That is correct, the current entity limit for applications is 100 for dev and dev pro
To get access to more application you can upgrade to an enterprise plan. If you have more detailed pricing and sales questions, or simply want to give the team feedback about pricing, please submit a request to the sales team via the “Talk To Sales” button in the nav bar.