You can merge your PST files into one by creating a new PST and then import all existing PST files into newly created PST file. However, if you feel that this process is lengthy and you want a quick method, you can use the PST Merge utility. This will allow you to combine PST files into one and remove duplicate items. It supports to join or merge PST files in new PST, existing PST and Outlook Profile. The tool is able to merge multiple PST files in Outlook 2019, 2016, 2013, 2010, etc.
Read More- https://www.systoolsgroup.com/pst-merge.html