It can vary vastly.
I‘ve been in agencies where I had one client but the responsibilities where more broad. From strategy to content creation and business development.
In other agencies we had L‘Oreal as a client and almost all of their sub brands as clients where we had a team of like 7 for around 25 brands just for one client.
I even saw some wild numbers on this subreddit where one person manages 30 clients.
Personally I‘m a fan of about 3-4 brands. This gives me variation in creativity, a greater sampling size to test out things and enough to do to fill 40-45 hours a week.