You use as many pixels as the total number of people who opened your email. For example, if you send 1 million emails and have an average total open rate of 10%, that campaign would use 100,000 pixels.
It is important to note that your pixel usage only accounts for total opens, not unique opens.
However, there are several factors that can influence the pixel usage, such as how many people have their images turned on.
Important: Whenever you send with the pixel, it counts against your allotted monthly pixel limit.