Question: I am calling a web service and the data from the web service is in csv format. 
If I try to save data in xls/xlsx, then I get multiple sheets in a workbook.
So, how can I save the data in csv with multipletab/sheets in c#.

I know csv with multiple tabs is not practical, but is there any damn way or any library to save data in csv with multiple tabs/sheet?

Answer: 
I've been using this library for a while now, 

<https://github.com/SheetJS/js-xlsx>

in my projects to import data and structure from formats like: xls(x), csv and xml but you can for sure save in that formats as well (all from client)! 

Hope that can help you,, take a look on online demo, 

<http://oss.sheetjs.com/js-xlsx/>

peek in source code or file an issue on GH? but I think you will have to do most coding on youre own
I think you want to reduce the size of your excel file. If yes then you can do it by saving it as xlsb i.e., Excel Binary Workbook format. Further, you can reduce your file size by deleting all the blank cells.