Kinda hard to answer without knowing lots more.

e.g.

1. will most BI reporting be country-specific? (argues for separation)

2. what are the expected table sizes? (if small-ish, no value in separating by country)

3. would your proposed DWs be housed in the same physical server farm or locally in the countries they represent? (argues for separation)

4. How costly will the ETL to the "master" DW be?