counter for countries
Used after a number to count countries, e.g., 3カ国 (three countries). The small カ/ヶ is a counter particle, not the katakana for 'ka'.
This treaty has been ratified by more than 50 countries.
Next month I plan to visit three countries.
Participants gathered from 20 countries for this event.
He has visited 30 countries for work so far.
カ国 counts countries while か月 counts months, but both use the same small か/ヶ counter particle and are easy to mix up in writing.
Standard spelling in modern Japanese; the small katakana カ is the most common way to write the counter か.
Hiragana spelling, sometimes used in less formal writing.
Variant with small katakana ヵ; less common than カ or ヶ.
Also very common; the small katakana ケ is read as か in this counter context.
Kanji spelling using 箇; now obsolete in this counter usage.
Full-size katakana ケ; occasionally seen but nonstandard.
The counter か/ヶ originates from the kanji 箇, which was used as a generic counter in classical Chinese. Over time, the character was simplified to the small katakana カ or ヶ, and it is now read as か in this context.