What does "days per month" mean?
For instance "50,000 days/mo" means you can analyse up to a total of 50 000 days in a month (any country, any number of requests).
When you send a request like "..command=analyse&start_date=2019-01-01&end_date=2019-12-31" , it will take off 365 days of your monthly quota because they are 365 days in the year 2019.
When you send a request like "country_code=FR&command=add_working_days&start_date=2020-01-01&increment=100", it will take off 147 days of your monthly quota because the resulting period is 147 days long.