A typical sitar has 18, 19 or 20 strings (depending on the style) It also has 11, 12 or 13 sympathetic strings or tarbs (A.K.A. "tarif" or "tarifdar" ) running underneath the frets.
The usual number varies with the instrument. Violins and ukuleles have four. Guitars usually have six, but some have as many as 11. Mandolins usually have 8 strings but some have 12 or 16. Banjos...