I'm new to wordpress. I want to create some main categories and 5 sub categories for each main category. Sub categories will be same for both main categories. E.g main categories will be "USA","Canada" etc. sub categories will be "places","photos" etc. Is it possible to make this with single wordpress or do i need to install another wordpress for each main category?
You may want to also consider using multiple taxonomies, not just one- categories. One taxonomy for location, another taxonomy for content type, then you won't have duplication of sub-categories. see taxonomies in codex for more info.
Yes it is, just create these categories from the categories menu in wordpress. Though, you need to create main categories and then subcategories within each. Thats how wordpress works by default.
So you create two categories
"USA" and "Canada"
and then create "places" and "photos" twice and then select different parents each time.
So you don't need multiple wordpress installations.