I do part-time online coaching (max of 20hr per week, but generally less) and have a max of 20 runners at a time, often less. If I wanted to spend 40+ hours per week I could handle many more but not necessarily double the clients because I would probably start mixing people up and making mistakes.
A lot of the time I spend "online coaching" is on developing content, marketing, logisitcs & admin stuff. If you can avoid doing that (join a coaching group that feeds you clients) I imagine it would free up a ton of time.
Clients take more time when they are new and you're learning their running style, their strengths, how they thrive. It gets easier/takes less time after months and years for sure. When you really know them.
Longtime client, independent type - 20-30min/week (minimum)
New client, lots of questions, 1-2 hours/week
Good luck, hope this helps a bit. Keep asking around I'm sure there are a lot of different answers to your questions.