src/include/pg_config_manual.h 

/*
 * Maximum number of columns in an index and maximum number of
 * arguments to a function. They must be the same value.
 *
 * The minimum value is 8 (index creation uses 8-argument functions).
 * There is no specific upper limit, although large values will waste
 * system-table space and processing time.
 *
 * Changing these requires an initdb.
 */
#define INDEX_MAX_KEYS		 32