Is there a quantity when buying wholesale products?

Quantity varies by supplier, most important is that you have a retail sales tax license or other credentials to support your wholesale status.

In general, there is a quantity involved, but it will depend on the seller, the buyer and primarily the product. Are you buying coal to fire a power plant? By the ton as sold out of railroad hopper cars? Are you buying milk to make cheese? In thousand gallon lots as sold out of a tanker truck? Are you buying barges to augment your fleet? In a one-lot from a marine builder?

There are many, many ways to buy and sell products and goods. And the wholesale cost and (minimum) quantity will vary by the nature of what it is that is being vended.