The number of projects possible in UFTree is limited only to your hard drive space. 10 GIG free should certainly be enough for most people. In fact, 1 GIG free would surely be enough for UFTree program and projects.
However, NEVER have more than one project open at a time. If you somehow do manage to have two projects open at the same time, one of the BAD things that can happen is that both projects become one which leads to many, many complications and perhaps even database corruption, which you don't want to have happen.
If you feel you need to have two copies of the UFTree program on your computer (and there are special circumstances when you might want this) be sure that the UFTree programs are NOT in the same folder. You still NEVER want both copies of the program open at the same time, either.
Should you desire two different copies of the UFTree program on the computer, AND if they are installed in different folders, you can still access any UFTree project from either UFTree program, but NOT at the same time.