I'm from the United States and there you hear both "clientele" and "clients". If you do a google search, "clients" gets 271.000.000 hits while "clientele" only gets 20.000.000, which is still a significant amount although much smaller than "clients".
I have a personal feeling, an intuition, that "clientele" sounds more posh or sophisticated to native speakers in the United States. Therefore, as always, the context that you use these words would help determine the best choice. If I wanted to sell diamonds or luxury cars, I would use "clientele", while for more normal, everyday business uses I would, and actually do, use "clients".
I hope that helps.
- For Teachers