Sort of a guess here that each file has the same columns - names and number of.
One item is to combine csv files. Tested in Linux without a problem using:
head -n 1 file1.csv > combined.out && tail -n+2 -q *.csv >> combined.out
This was found on this post → Merging contents of multiple .csv files into single .csv file. See comment under answer by
For Windows 10 here is a post → How to Merge Multiple CSV Files Using Command Prompt. Have not tested this.
Now determine where you want the data. If it is to go to a database, you need to choose which database to use. Importing this amount of data is critical in the amount of time needed depending upon this choice. For an example using 81K+ records, see this post → How to import a big amount of data into LibreOffice Base.
So depending upon the choice of residence of the data, you may want to have multiple files so 400K+ records is not a very, very, very long import task.