I can't give you the full US, but I can give you BNSF:
As of exactly right now...
BNSF has 223,827 cars on the system. That's cars of all owners.
BNSF has 5,882 active locomotives on the system. That is excluding those off-line or inactive due to various laid-up statuses -- and there's a LOT of those thesedays.
Do the math and that's roughly 1 locomotive for every 37 cars.