There is no hard limit that I am aware of, and there is none documented in the service quotas and limits or the Entities documentation pages.
However, there is a practical limit on this kind of extraction. The model in question is putting out a reasonable prediction:
- First, that a word or group of words is an entity
- Second, which of the OOB + Custom entities it belongs to
That second part is the tricky bit because the more entity types that it could match, the higher the likelihood that you'll 'confuse' the model and get inaccurate results. This is an inescapable statistical reality. So, if you keep adding a whole ton of custom entities what you'll eventually find is that the model accuracy degrades. There's no way to say when or how fast, but eventually you'll start to see the performance drop.
So, again, there is no hard cap I know of, but if you're thinking of trying to use this to extract hundreds of distinct custom entity types, this is not the right solution for you.