Should I enter all rental cars separately and by company or should I use one sum for each company. I used different cars with each rental company.

You do not need to list vehicles that you do don't own on your tax return.   You can report rental expense (for vehicles or equipment) for the portion of the rental expenses that are associated with your business income.  Keep detailed records and report the total rental expense for the year.  

On a Sch C the IRS only sees and wants totals in the categories .... the option to put in details in the program is for your use only and is not required (and is a big waste of time if you keep good accounting records).