What is determined by the number of congressional districts?
It’s kind of the other way around: the number of members of The House of Representatives is set at 435, so there are 435 Congressional districts. Because population has increased, Congressional district boundaries are periodically re-drawn to reflect a (supposedly) ‘equitable’ representation of the population, and the demographics of the population therein, usually after the every-10-year National Census is done. And because of the political nature of redrawing those boundaries, a lot of pressure is generated in the various state legislatures to push and/or pull the boundaries around to someone’s advantage ( =usually the Rep who currently holds that seat). That led to “gerrymandering,” the reconfiguration of a district to suit one’s desire to retain an advantage, usually to the detriment of the people in that district. Why not Google “gerrymandering” for an historical explanation.
