9 instruments track only?

I don't know if there is a limit on the number of Instrument Tracks you can have, but I think SHS 7 has no limit on the number of soft synths you can use. I could be wrong, though.
Try adding the soft synth normally instead of as an Instrument Track.
 

When you insert the Synth, instead of selecting Simple Instrument Track, select MIDI Source Track, First Synth Audio Output, and Synth Properties Page. This is what Greg is talking about.
Among other things, I think it gives you more flexibility and control. If you are using multi-channel synths like TTS-1, Sample Tank, etc., SITs are not your best option.
SHS7 has unlimited MIDI tracks, but it is limited to 11 soft synths. Even X2 Producer is limited to 20.
Now, if you are using a multi-channel synth, you have the possibility of 16 separate instruments in one synth.
That means up to 16 MIDI tracks feeding one synth.
You also have multiple audio outputs. TTS-1 has 4, and Sample Tank has 16 available.
A Simple Instrument Track does not take advantage of that potential.
For a one instrument synth, such as Studio Instruments Drums, SITs are ok.

My guess is that you have deleted some SITs, but left the associated synths in your synth rack.
Deleting the tracks just unplugs the I/O. It does not delete the synth. The synths must be deleted from the synth rack.

Which synths are you working with?

64 audio tracks, yes.
Unlimited MIDI tracks, yes.
Unlimited synths, no.
I just tried MC5(same generation as SHS7). It officially supports 8 synths. Attempting to insert the 9th synth, the menu was greyed out.
SHS7 was significantly downgraded from SHS6 in many ways. I just inserted 16 synths in SHS6, and it was still going.
Granted, that is way more than my system can handle.
Point being, when the menu goes grey, you are capped out.