Some very basic units have a single or dual channel scanning receiver that can scan and 'lock' on 8 to 12 satellites. This article answers the questions regarding 12 channel receivers and why you might want one. This is the first question you need to ask since there are two different interpretations of the answer. One is, any receiver capable of tracking 12 satellites. The second answer is a receiver having dedicated hardware to receive 12 channels simultaneously, also known as a twelve channel parallel receiver.
Today there are basically three kinds of receivers.
Multiplexing receivers, sequential receivers, and parallel receivers. There are also mixed types where some channels are dedicated to a single satellite and some channels are multiplexing or sequential. The receiver has dedicated separate hardware to receive each satellite that it needs for a solution. This kind of receiver uses a single, dual receiver, or even 3 receiver hardware design to time division multiplex among the satellites that it is viewing. This mean it gathers some data from one SV for a slice of time and then switches to another SV to gather some more data.