Something like this should do the trick:
{% set usersGroupedByCountry = craft.users.limit(null)|group('postaddress.country') %}
<ul>
{% for country, users in usersGroupedByCountry %}
{% if country and users|length %}
<li>{{ country }} ({{ users|length }})</li>
{% endif %}
{% endfor %}
</ul>
Note that if you want to order the list of countries, you'll need the SuperSort plugin. Here's how you would sort the list alphabetically:
{% set usersGroupedByCountry = craft.users.limit(null)|group('postaddress.country)|supersort('ksort') %}
Also note that if you have a lot of users (say, hundreds), it is probably a good idea to wrap the whole thing (including the {% set usersGroupedByCountry ... %}
part) in a {% cache %}
tag pair, for performance reasons.
country
field on your Users field layout? If so, what type of field is it?