counter
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'.
この条約は50カ国以上が批准している。
This treaty has been ratified by more than 50 countries.
来月、3ヶ国を訪問する予定だ。
Next month I plan to visit three countries.
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.