When Sirius XM counts its number of subscribers, I don't think it counts unsold cars sitting on the dealership lot. When you buy the car, the Sirius XM radio only plays the Barker Channel 184. It gives brief promos of shows and channels, along with frequent mentions of ways to activate your radio. Upon activation, you get at least a three month free introduction with all the available channels.
I'm not sure if you have to give your credit card info at this time. But obviously Sirius XM wants you to continue as a customer when your trial wraps up. With my latest car, after the three months were over, the silent radio one morning said "Updating Subscription 10%, 20%, etc." for about 30 seconds. After that, you only get the channels you pay for. I assume if I hadn't already signed up for the service, it would have gone back to only the Barker Channel with no subscription.
The new CEO is considering a free advertiser-supported service of limited channels. But that hasn't been implemented yet.