I contacted Collin Biedenkapp from TFDi Design about this already some time ago, and although he was aware of the issue, he could not explain himself how this "limit issue" occured.
So yes, to answer your question, apparently there is a bug there, so what i did on my PC is the following:
I created a subfolder, where i keep most of my soundpacks "hidden" from the 'active' soundpacks folder. Let me show you what i mean:
The folder "soundpacks" is the folder where Smartcars2 reads the actual active soundpacks from, so i keep a maximum of 5-6 there, to avoid overload.
The subfolder (here called "= MORE" which i keep "on top" with the '=' sign) is where i keep all my other Soundpacks. It really just takes a second to copy and paste it in the "active" soundpack folder, for it to be recognized and used by Smartcars2. This is my own workaround, nothing endorsed by TFDi Design.
FYI: The Installers that i create and that you can find in our Download section, will point to the standard "soundpack" folder, but you can move them afterwards out of it if you installed more then 10 or so.
Hope this answered your question and helped you understand a bit more on how Soundpacks/Smartcars work.