You can try something like the following at the very top of your .htaccess file in the document root (before the WordPress front-controller, ie. before the # BEGIN WordPress block):
RewriteRule ^([\w-]+/)wp-admin /$1 [R,L]
This will redirect /<site>/wp-admin to /<site>/ – where the site name can consist of any of the characters a-z, A-Z, 0-9, _ and -.
http://www.example.com/wp-admin is accessible as normal.
Yes its working, if wordpress is in sub directory its not working.
If the WordPress .htaccess file is in a subdirectory then you will need to modify the directive like so:
RewriteRule ^wp-admin /subdirectory/ [R,L]
However, your setup is still unclear. Do you have multiple .htaccess files for different WordPress installs? A single multi-site .htaccess in a subdirectory and no other .htaccess files?