Description
https://w3c.github.io/clreq/#id69
Current text:
In principle, if numbers are arranged horizontally in a vertical writing mode, the width of the numbers should not exceed 1 em. This rule originated in the letterpress printing era due to the fixed width of each line. Therefore, in vertical writing mode, Western text or European numerals are not limited to two digits, but the width should not exceed 1 em. Some commonly seen examples include "3.0", "A+" and "2B".
Suggestion from @r12a:
In principle, if numbers are arranged horizontally in vertical writing mode the total width of the numbers together should not exceed 1 em. This rule originated in the letterpress printing era due to the fixed width of each line. The horizontal text may include more than two characters, but the overall width should not exceed 1 em. Some commonly seen examples include “3.0”, “A+” and “2B”.
Another comment: jlreq mentions that font glyphs can be squeezed to fit more in, and the limit is usually 4 characters. clreq doesn't mention this currently.