I am wondering if "some customers" can refer to a specific group of people at the restaurant in the following context. Does it need to be changed into "Some of the customers" or "Some"?
There are many customers at this restaurant today. They come from different countries. Some customers prefer coffee. Others prefer tea.
It depends on what you mean by "Some customers prefer coffee. Others prefer tea."
As it stands, your last two sentences do not refer to the customers in the restaurant today. They refer to customers in general.
If you want it to mean, "A subset of the customers at the restaurant today prefer coffee, while a different subset prefer tea", then you must say either "Some of the customers prefer coffee" or "Some prefer coffee".
Why? Because you have already introduced "customers" in the first sentence. After that point, every time you refer to them again, you must call them "the customers". If you don't, it sounds like you're referring to a different set of customers. The set of customers might be "all customers who ever come to this restaurant" or "all restaurant customers generally", but without "the" it cannot mean "the customers at this restaurant today".
"Some prefer coffee" works because after the introduction of a group, "some" means "some of them".
