To mix more than four tracks together in MuLab, you’ll need to use the same trick the Beatles did when making the Sergeant Pepper’s Lonely Heart’s Club Band album with 4-track tape recorders.
Once you get up to four tracks (but not more than four), and you have something that you like (with all the panning, volume control, etc.), save that session, then choose Mixdown Audio from the Session menu to mix what you’ve done so far down to a single stereo file. Then close that session (quit MuLab entirely and then reopen it), open a new empty session, and drag your mixed-down audio file into your new session. That will be one track, but you’ll now be able to add up to three more tracks to it. (And if you decide you want to change something about the mixed-down audio, you can always go back to your first session, make whatever changes you need to make, then repeat the Mixdown Audio process to get a revised version.)
You can repeat that process as many times as you need to, in order to mix together as many sounds as you want. There’s no loss in sound quality because it’s rendering everything exactly as you specified it in your original session.
Track 1 — \
Track 2 — \ ___mixed down to … Track 1 — \
Track 3 — / and add … Track 2 — \___mixed down to … Track 1 —
Track 4 — / Track 3 — / and add … more tracks, etc.
. Track 4 — /