You can put a lot more then 6 charts on a sheet, but the more you put on there the smaller they will be.
You should not aim at pushing as much visualizations as you can on a sheet but instead only what is needed for a specific topic or question. Make a new sheet for other questions / visualizations; you don't pay licenses per sheet anyway.
In other words:
Less is More.
If I put charts (line charts) that can be seen well, not small, 4 of them is the max number.
For example, when you want to analyze the daily user messaging activity in an app, it takes much more than 4 charts. It would be great to have the option to put even 20 charts, one beneath the other, without moving to a new sheet.