Open the SYNTH RACK view.
It's very likely that you have 8 synths in the rack.... perhaps you tried some others and removed them form the FX bins or deleted the tracks with the synths in them. Doing that clears the synths from the tracks but leaves them still loaded into the project.
The only way to check for sure is to open the synth rack. There, you can see exactly what is and isn't loaded and you can remove the ones you don't need to make room for the ones you do need.