The 18650 descriptor refers to the physical size of a cell.
18650 = 18mm x 65mm
26650 = 26mm x 65mm
18500 = 18mm x 50mm
Etc.
Generally, larger size means greater capacity.
This -
https://www.reddit.com/r/electronics...hts_and_album/ - is a tear-down of an older Valence U27-12XP (black case).
He found 400 x 18650 type cells inside. 4 batteries x 400 cells = 1,600 cells. Newer Valence U27-12XP might be different.