The C major thirteenth chord is a 6-note chord consisting of the notes C, E, G, B, D and A.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Cmaj13.
Chords that are a subset of Cmaj13. These contain less notes but all of them are included in Cmaj13.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Cmaj13 chord. Feel free to save or share the image as needed.