How the total quota limit is applied depends on the
type of Properties Service being used.
If it's document properties then the total limit is for one document.
If it's user properties then the total limit is for that one user.
And Script Properties is for one Apps Script file.
There are various other unique situations. Web Apps do not have Document Properties, they are not a document.
There is rate limiting that applies to how often the property can be accessed in a short period of time.
The exact rate limiting quota is not documented as far as a I know. You'd get an error message. If you get error messages then you need to write code to wait/sleep and then retry.
The key length for the property name is limited to 250 Characters.
Cache Service and Properties service have different total limits.
Use the JSON utility to stringify and parse JSON for conversion.