Usually there are some block of flight numbers which later on are added to same country flights. For example TK1 is JFK
, TK10 LAX
, IIRC TK5 is ORD
and so on. TK
20,27... are for China, and so on.
For example TK1853/4/5/6 are BCN
flights, and later on this year new flights were added as 1851/2, as they were unused.