What is the best strategy for making a group of wordpress sites to target multi-countries
Recommendation: Hire someone smart who can help you do this.
You must be very careful about how you approach this.
Based on doing this many times, it takes a massive amount of effort to fix this type of content organization after the fact, because you may end up having to recover from SEO penalties.
Far better to simply design this correctly from the start.
1) You have far to little information to provide a solid answer, so guessing is required...
2) What you may mean, guessing, is you have either common content or related content, which is multilingual.
3) If #2 is correct, then you must never guess about how to do what you're asking.
You'll either dig into Google docs about how to correctly arrange all your content across a single site (common content in different languages) or correctly organize related content.
Big Note: If you make the mistake of creating exactly the same content, in different languages, on different sites... what tends to occur over time is Google figures this out, then as time goes by some posts for some countries trend (depending on which country is indexed first), then other countries are penalized as duplicate content.
Then, far worse, if one site, say a German site has to high a percentage of page level duplicate content penalties, then the site gets a site wide duplicate content penalty.
All this is extremely simple to design from day one.
Trying to recover from page or site penalties in the future can be near impossible, because once penalties are levied (page or site), SEO recovery takes a very long time.
As I said above, hire someone to help you with this.
Tip: You'll know you have the right person for this project, when the rate they quote makes you weep. :-)
I would recommend one of the Multilingual plugins for this (I use Polylang most of the times) if the content is the same for all languages and you need just translation.
But if you are thinking of having different websites targeting different markets I would use WordPress Multisite to build a network of websites.
It can be a network of subdomains for each language/country (en.site.com, es.site.com, de.site.com) or Top Level Domains (site.us, site.de, site.es etc).
Check out my talk about using WordPress Multisite for these types of cases https://sabrinazeidan.com/wordcamp-london-2018/ (Starting 20:41 in this video)