An Outlook user with quota on exchange server must back up email. Consider email archive going back from 2013 to 2002. What is the optimal way to manage and back up your email using PST files in the long run. What filing structure is optimal for storage and searching (max limit for PST files).
In Outlook 2010 and 2013 the size limit for .PST files is preconfigured to 50 GB. Some filesystems have a general 4 GB size limit for files.
Example: To do a .PST backup on an USB stick, this stick has to be big enough and formatted with NTFS/ExFAT if you have a .PST exceeding 4 GB.
There seems to be no hard limit for the number of items per folder. But I would recommend not to go beyond some 2.000 items. Otherwise, searching and navigation become quite slow.
The "filing structure" can either be content-based or time-based. Content-based structure means to separate your files according to customers, projects, topics, technologies, ... This is helpful for manual navigation, but it takes time to sort all files initially. Another drawback is that files might belong to more than one category.
With the advent of powerful search facilities, the time-based structure has gained followers. Depending on the number of incoming/outgoing messages per period, it makes sense to organize the files in days/weeks/months/quarters/years. The constraint is the maximum number of items per "bucket" mentioned above. I keep separate folders for inbound and outbound messages.
Depending on your e-mail traffic an archive from 2002 to 2013 might require multiple .PST files. It usually makes sense to only bind the most recent .PST and leave the elder ones unconnected. Obviously, a forgotten .PST password would render the .PST file useless.
I am not certain about one special point: Imagine you are storing encrypted mails over many years. To actually read such mails, you'd need the private key which was valid at creation time. Make sure you get this sorted out. But I don't know how.
Apart from technical criteria, the working requirements might influence the filing structure. Special folders for "work in progress" or "important" items can help to organize the daily processes.