In Kart Racing Pro, the basic set for maximum number of karts in your drawing distance is 5. To set the number to KRP's maximum of 12 in total, you need to modify your profile.ini file. This can be found within your profile's folder, as shown in this picture with "Xander Clements".
Next up, you need to open the file "profile", which will either have "profile.ini" for those on Windows XP or lower, or "profile" and configuration settings as the file type for Windows Vista or higher.
Once this has been opened, you need to scroll down to "[net]" and change the connection from "connection=0" to "connection=2", as seen in this picture.
Once this has been done, left-click file and then on the dropdown select save. Once done, close the file.