how many installation needed for many subdirectory websites?

Hello,

Thank you for reaching out to us with your question about installing Rank Math on multiple subdirectory websites under your main domain site.

For your setup with a main domain site like nubestskin.com and subdirectory sites like nubestskin.com/au, nubestskin.com/us, nubestskin.com/ca, etc., you only need to install Rank Math on the main site (nubestskin.com).

When you install Rank Math on the main site, it will cover all the subdirectory sites as well. You do not need to install Rank Math separately on each subdirectory site.

If you have any more questions or need further assistance, please feel free to ask.

Thank you for using Rank Math!

Best regards,
Rank Math Support Team

Sources:
Installing Rank Math on a Multisite Environment » Rank Math
How to Install Rank Math Using an FTP Client on Your WordPress Website
Does Installing Rank Math Slow Down My Website? » Rank Math
What Are the Best Robots.txt File Rules for Multisite? » Rank Math