If you want to put 7gb of ram put it, it is put in pairs to take advantage of the dual channel but it is not necessary it was an old standard to improve the performance and to make work the bus of the ram to full, mine has a bank of 8gb and another of 4gb ddr4 4000mhz with so much speed of bus the dual channel is unnecessary

There are, sometimes they come with 2 slots of 8 and 4.

Now the issue really is that they make them with 2, 4, 8, etc. This is because memory uses binary. 1 byte is 8 bits. Ones and zeros. And each field in memory is represented in this way. So manufacturers make memory based on powers of 2, which results in 2 4 8 8 16 32 and so on.

Because of how they work as they are multiples of 2 as the information in the computer is binary so you will find memory sticks not only ram but for example USB of 2,4,8,16,32,64,128,256,512,1024 etc ...

A quick and simple example. The smallest unit of information in the bit, which can be either 0 or 1. That is, the options we have for a single bit is 2^1 i.e. 2. We know that a GB is 1000 (or 1024) bytes which in turn is made up of bits. So for N number of bits we have 2^N (two to the N).

Hmmm well you see it's up to the manufacturer, it depends on the design, available memory, BIOS, number of slots. There is no universal reason. And if there is with that amount of 12 Gb. I have an Hp that I bought last year AMD.

My laptop for a long time had 12 GB of ram, because it had 8 gb that came from the factory and 4 GB that I put later, about 6 months ago I managed to put another 4gb Sodimm and finally I raised it to 16 GB.

You can put 1,2,3,4,5,6,8 or however much memory you want in your computer as long as you have the modules to do it or the computer supports the memory in configurations that don't demand all the modules to be the exact same size.

You can even have 1.5, 2.5, 3.5 or more GB if you have the required modules. For example one 1g and one 512mb and so on.