Everything has a limit.... just that some are higher then others.
Freezing a synth doesn't remove it from the synth rack where all the synths in the project are installed. In MC, that rack has room for 8 VSTi's .... frozen or not.
That's why bouncing to audio is the workaround solution. That frees the connection between the synth and the audio.... so you can now use that synth for a new track.
Remember that you can do this: After the bounce, simply change the output of the midi source to NONE. then "hide" the midi track. You can always unhide it if you need it back. The synth stays in the rack until you manually delete it, so you can simply add a new midi track, set it up to work with the synth and you're ready to go on the next track after changing the patch or the sample the synth will play in the new track. I can't tell you how many times I have done that very thing.....