Select samples from a dataframe in python
If I understand your question correctly you can break this problem down into two parts:
1. Figuring out which country occurs most frequently and then
2. Subsetting the pandas dataframe to that country

import pandas as pd
from scipy.stats import mode

# 1
mock_df = pd.DataFrame([{'country': 'a'}, {'country': 'b'}, {'country': 'a'}])
print(mock_df)
#   country
# 0       a
# 1       b
# 2       a
mode_country, _ = mode(mock_df.country)

# 2
mock_df[mock_df.country == mode_country[0]]

#   country
# 0       a
# 2       a
```