According to Serotonin in the Gastrointestinal Tract Current Opinion in Endocrinology, Diabetes and Obsesity (2009) volume 16, pages 53-59:
95% of the serotonin in the body is located in the gut
Furthermore, this does not mean the rest is in the brain. Serotonin is also in platelets. There is a nice explanation in Relationship of Platelet Serotonin to Disturbances
of Clotting and Hemostasis Circulation 1957; 15:353-357
Janewayand coworkers
found that platelets were essential to
the vasoconstrictor activity of serum. This
observation was later confirmed by several
other investigators. In 1948, a crystalline
vasoconstrictor substance was isolated from
serum by Rapport and associates and
named "serotonin." This substance, subsequently
identified as 5-hydroxytryptamine,
is derived biologically from tryptophan. [references omitted]