The Union Pacific, a railroad known for having too many locos, has 8,000, and 64,191 freight cars, or 1 to 8. BNSF has similar 8000 locos but almost 90,000 cars, or 1 to 11.
BUT these locos do not just move railroad owned cars. I am guessing that fewer that half the cars on the system at any given time are their own. Think of all the other railroad cars in interchange. Seven class ones interchanging equally would yield some challenging calculations. Then all the private companies out there (chemical companies UTLX, general transportation ABOX, all the power companies APOX, grain companies CGCX, etc. ). Simple on-line searches have not been helpful. I am guessing the ratio is at least to 1 to 20. Someone should do some research and write a scholarly work to submit to the model railroad related rags.
And then of course there is length of rail to loco ratios to talk about/consider. For me with zero rails the ratio doesn't even work since division by zero is undefined.