The B flat major thirteenth chord is a 6-note chord consisting of the notes Bb, D, F, A, C and G.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bbmaj13.
Interactive piano diagram for the B flat major thirteenth chord
Chords that are a subset of Bbmaj13. These contain less notes but all of them are included in Bbmaj13.