I know that the older versions of MC did in fact have a limit of 8 synths at a time max. Plugs and FX on the other hand were not limited except by the computer's power to process them all in a timely manner.
Perhaps the bakers decided to bump up the max number of active synths permitted in MC7... I don't know because I can't run it on any of the computers I have...OS issues.
However, if you are bumping into the 8 synth max ceiling, there are ways around it.
1. Render the synth output to audio and remove the synth from that track.... This saves the audio and then allows the synth to be used for something else. NOTE: make notes on the synth and any custom settings and the sample you used in case you do need to go back and rework something later.
2. Use TTS. Each individual instance of TTS is capable of handling 16 discreet channels of midi with a different patch (sound) applied to each one. It isn't a coincidence that MC can handle 128 midi tracks. With TTS in all 8 synth slots, you could theoretically have a 128 piece orchestra in a project. If you're not sure how this works, look at my website under the recording tab and look for TTS Advanced setup. It's really not a difficult concept if you understand MIDI already.
3. Upgrade to one of the Sonar DAW's that don't have limits on synths.
Hope this is useful to you.