@simonferndriger said:

When using https://api.themoviedb.org/3/tv/2288/credits?api_key=### I'm getting (the top) 10 entries for the cast.

With this call, as explained in the documentation (the big note in blue), you get the regular cast of the last season. It's a coincidence that it matches the top 10.

But on https://www.themoviedb.org/tv/2288-prison-break/cast there are listet 193. Why the difference?

To get the complete cast of the series, you need to use https://api.themoviedb.org/3/tv/2288/aggregate_credits?api_key=###