Mixcraft 8 has a "Mix Down to Stems" option on the File menu. Download Mixcraft 8
There's no set limit to the number of tracks you can mix down to in Mixcraft 7 (assuming there's plenty of hard drive space, etc).
Make sure you've got Mixcraft v7.7 build 316. If it's an older build, choose "Check For Updates" from the Help menu and then quit Mixcraft as the patch downloads in order to install it.
If that's not the problem, try it again and send us the Mixraft-log.txt file.
To find the mixcraft-log file, highlight the following line and press Ctrl+C to copy it:
Press the Windows Key+R to pop up the Run box in Windows. Click in the Run box and press Ctrl+V to paste in the line from above. Finally, click OK to go to the location of the mixcraft-log.txt file. Submit an Acoustica Support Request
and attach the log file along with a brief description of the problem.