number of characters; character count
Used for counting letters, kanji, kana, or any characters in a text.
This report needs to be at least 2000 characters long.
Count the number of characters in a tweet.
Please count the number of characters in this report.
This form has a character limit.
The message exceeded the character limit.
Please keep the headline within 20 characters.
文字数 is the general term for a total character count, while 字詰め specifically refers to the fixed number of characters set per line or page in a layout.
文字 means 'a character/letter' itself, while 文字数 means 'the number of characters' — easy to mix up since 文字数 contains 文字.
characters per inch; CPI
Compound of 文字 (moji, 'character') and 数 (sū, 'number').