Normal capacity on your standard issue 737-800 airliner (note: airliner) will be 20820 kg - you will notice that this is the lowest capacity listed.
20820 KGs is what you can load to the wing tanks (just shy of 4 tonnes each in NG
) plus center tank (13 tonnes)
Now why did I say to note airliner - the table includes BBJ2 variant, that is a common user of auxiliary tanks.
You can install several of those (up to 9) in your baggage holds, sacrificing baggage space (not really needed on BBJs anyway). Those will take you to the other fuel capacities for the BBJ
, up to a maximum of 31.7 tonnes.
You will see on the graph that they become useless after certain amount of payload, as you cant fill them with fuel anyway as it would take you over maximum take-off weight of 79 tonnes.