In Outlook 365, there is a limit to how many PST files can be opened at the same time. The maximum number of PST files that can be opened in Outlook 365 is 15. This means that if you have multiple archives and want to open all of them in Outlook, they can open up to 15 PST files simultaneously.
Please note that this limit of 15 PST files specifically applies to Outlook 365. Other versions of Outlook may have different limits. Moreover, if a large number of PST files are opened at once, it may affect the performance of Outlook.
If you need to access more than 15 PST files in Outlook 365, it is advisable to consider consolidating or merging some of the PST files to reduce the number of open files. This can help maintain optimal performance and improve usability in Outlook 365.