The Maximum you can collect a Day is 50 coins, Full Stop no more is possible.
In order to do this 2 things must happen.
Collectively the time one or multiple Pokemon must defend a Gym in a 24hr block is just a bit more than 8hrs, I think that time block goes from Midnight to Midnight.
The Pokemon needs to be battled out of the Gyms and come back to you to collect. (no return no collect)
You could have one in there for just more than 8 hours, get booted and you get 50 coins
You could have 4 Pokemon in there and all get booted over the course of the day. The hours they spend in there collectively adds up to just more than 8hrs you get 50 coins.
You can only be in a Maximum of 20 Gyms at a time.
If you have multiple Pokemon in Gyms and nothing comes back in the 24hrs you get nothing on that day. If they booted 2-3-4 days or even weeks later you still only get 50 coins once returned if you haven’t already collected for the day.