I was not aware of this limitation up to now because of having never opened more than 3 compare sessions at the same time in same instance of UltraCompare.

There is definitely no setting to configure the maximum number of compare sessions.

So the only workaround for this limitation is checking in Configuration at Application Layout the setting Allow multiple instances and start one more UltraCompare instance for using more compare sessions at the same time.

Well, I expected that reply already because I thought the multiple sessions were caused by running a folder compare and double clicking on lines to run file compares for the files in each double clicked line. But I thought also why a user should have more than 1 to 3 file compares open in addition to a folder compare. That does not make much sense in my opinion as it is just confusing to do so many compares parallel at the same time.

However, it is of course possible to run in second instance of UltraCompare the same folder compare and double click there on other lines.

But if you want to know the reason for a maximum of 10 sessions, you need to ask IDM support by email.

I suppose the reason is saving the sessions for automatic reload on next start with 0= to 9= in Windows registry or uc.ini.