multiple wordpress installation on a single domain

Why not create a sub-domain and install wordress there instead. If you are doing it on the same domain it may likely create some problems although that statement is not verified.
But say, the main domain is https://www.wordpress1234.com. Create a sub-domain like say https://www.client1.wordpress1234.com and install a wordpress there. You will get a different wp-login for that.

or you could use a single install with multisite

https://codex.www.ads-software.com/Create_A_Network

Each site acts independently, each can have it’s own theme, etc.

Otherwise, you can install in subdirectories (domain.com/A) or subdomains (A.domain.com) as many times as you wish. You can use differnt databases, or the same database specifying a different prefix for the DB during install