I wrote a Python script to get the total amount of posts from each country on /bant/ from https://archive.nyafuu.org .
There were 5974542 total posts when I ran the script, and it returned 5193833 posts. Since you can't search for imptrash on Nyafuu, that means 780709 posts, or 13% of /bant/, is just imptrash.
2202185 (36.86%) of posts have come from America alone, and America more posts than the next 18 countries combined.
1564933 (26.19%) of posts come from Europe, the UIK being the largest contributor with 355181 posts.
The 57 countries under the Africa ISO code total 38639 posts, or about 0.65% of /bant/.

There are 7 countries (excluding A1, A2 and O1) that haven't posted on /bant/. These are:
Bouvet Island
Western Sahara
South Georgia and the South Sandwich Islands
Heard Island and McDonald Islands
British Indian Ocean Territory
French Southern Territories
United States Minor Outlying Islands

And finally the five biggest countries in order are:
America with 2202185 posts
Canada with 370265 posts
England with 355181 posts
Australia with 193829 posts
Germany with 166757 posts