NLS_CHARSET_DECL_LEN returns the declaration length (in number of characters) of an NCHAR column. The byte_count argument is the width of the column. The char_set_id argument is the character set ID of the column.
The following example returns the number of characters that are in a 200-byte column when you are using a multibyte character set:
SELECT NLS_CHARSET_DECL_LEN(200, nls_charset_id('ja16eucfixed'))
FROM DUAL;
NLS_CHARSET_DECL_LEN(200,NLS_CHARSET_ID('JA16EUCFIXED'))
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100