I have checked from my side and tried to reproduce the issue you are facing.
However, I was successfully able to import 5/10/15/20/30 companies on different occasions.
Can you please tell me whether you are facing any issues after trying to import 5 companies at a time?
This problem might be caused by an internal issue with your server.
So, please create a support ticket through link: https://wperp.com/contact/