RCS model - Question on number knots

Dear Colleagues:
Interested in your thoughts on the # of knots to select on an RCS model. In this body of work, we are using RCS to fit a concentration curve between 2 biomarkers and calculate a ‘cut-off/threshold’ of our x-variable/biomarker.
Five (5) knots have been okay for our analyses thus far and our sample sizes have been pretty robust (often >200 observations), but we keep getting questioned why 5 knots, and not 4, or 8 etc.

Below are 2 fits and the “threshold” concentration for serum ferritin against hemoglobin. Our model is also guided by physiologic interpretations, so we are interested in the concentration at the first inflection plateau. We achieve different SF concentration depending on the knots selected, and we don’t want the whole process to be data driven.

Do you happen to have any thoughts on how to select on the best number of knots for a dataset?

Thank you for your time and insights.

Ps. RCS fit for US women data, and physiologic ferritin ‘thresholds’ derived from Function (RCS.fit), c(5,50).

Please move this post to RMS General Regression - modeling strategy - Datamethods Discussion Forum after reading other topics there to see if this is already covered. Thanks.