What should I expect to pay for a taxi to Atocha train stain from the airport two people and 4 pieces of luggage? Thanks

You should expect to pay whatever shows on the meter (probably around 13-14 Euros) plus the 4.20 Euro airport surcharge. There is no longer a surcharge for luggage (though 4 pieces might not fit in the trunk of a standard taxi and you might need to request a mini-van or two taxis). Depending on how fit you are and time of day (avoid rush hours), you can also do this trip by a combination of Metro and Cercania train or Metro alone.

As Eduardo suggests, you can take the subway from the airport to "Nuevos Ministerios" and the the commuter train from there to "Atocha". It should take you about 25-30 minutes. Perhaps this is even faster than a taxi.

By the way, if you have the train tickets by the time you arrive at Nuevos Ministerios, I've heard you can travel for free on the commuter trains from Nuevos Ministerios to Atocha just showing them. It seems they are kind enough to let you go for free to your departure station. However, I've never tried myself and I'm not sure whether this system is still valid.