I would never use the word "clientele" to refer to a single person, but probably only to refer to the whole (or at least the generality) of the clients of a particular business or group of businesses. And I would never use the word "clientele" in the plural.

clientele is a synonym of clients (plural) online Collins dictionary clientele SINGULAR NOUN [with singular or plural verb] The clientele of a place or organization are its customers or clients.

A clientele is a customer who is loyal to the shop or restaurant and frequently goes there. And a client is just a normal customer.