If you see ev-cpo, there are just 488 Model 3s in US. That is less than 1/2 day worth of sales ! So, they are clearly very low on 3 inventory.
500 is low for a model selling some 10k per month.
Ok, then I'll rephrase. For the entire life of the Tesla's (previously CPO) inventory website, they've never listed all of their available inventory for any model, new or used. So if there are 50 cars on EV-CPO or 500, that's just because Tesla decided to list that many online. It really has no bearing on the actual number of Model 3s in inventory available for sale. It could be 500, or 5000, we really do not know. It's up to the remarketing department to decide what cars they want to list, and how many. They always hold back cars that remain unlisted.