Yes and no. It means you can no longer interface with that satellite as it will never accept your node ID again. (Unless reentry is implemented at a later moment)
You can still join other satellites, but currently there are only 6. So for now you would be stuck with 5, yes.
Yes, they have 6 at the moment. Satellites have been renamed. Us central is now us1. This is because they were migrated into multiregional setups and because the domain changed from tardigrade to storj. In the past there was a stefan-benten satellite as well, which has been decommissioned (though work is ongoing on reintroducing one on that domain).
The current active satellites are listed here. https://www.storj.io/dcs-satellites
Unless you exited one or were disqualified, your node will be connected to all of them.
Your node is not limited to connect to only 6 satellites. If a new one goes life on the satellite list I just posted, your node will automatically connect to it. I’m not entirely sure if you even could add test satellites to a node that is also using a trust list at the moment. But I wouldn’t recommend it. The idea with the test satellites is that you can test things from both ends. So we as node operators can also test to see if we can mess with the satellites 
. It doesn’t seem like a good idea to mix test nodes with production nodes.
You can set up a new node to join the test satellites though.
Info on that here: Please join our public test network
And here: Stefan Testnet Satellite