Just thought that I'd point out a few other aspects that you might want to consider.

Although it looks like you're wanting to reduce the CSV file by limiting the number of lines in each new CSV file (which will become rows in Excel), I thought that I'd point out that Excel has a max number of both columns and rows that it can handle. The max number of columns and rows supported will vary across versions of Excel. The reason for bring up this point is that if your original CSV file has more columns than what is supported by Excel, all of the split CSV files that you create from the original will have the same issue when trying to open in Excel.

Also, does the first line of the CSV file contain the column headers? If so, you'll want to capture that first line and reproduce it as the first line in all of your new CSV files. Otherwise, only the first new file will have the headers.