http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_in

SELECT val1 FROM tbl1 WHERE val1 IN (1,2,‘a‘);

The number of values in the IN list is only limited by the max_allowed_packet value.

http://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_allowed_packet

You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 1GB. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.

max_allowed_packet