I want to toast some peanuts in a microwave (just a microwave - it does not have a grill (broiler) or convection mode). Can I put multiple layers of peanut in a container and toast it? I am not sure as many places I read, they say to toast single layer. But why should that matter?
Microwaves don't toast.
The pass high energy microwaves through food. These waves struggle to penetrate high density materials, causing them to heat. It's (mostly) the water within the microwaved food that is heated up, and that cooks (or defrosts) your stuff.
Toasting is an external application of usually radiated (but conducted will do) dry heat.
You might be able to heat the oil within the peanuts to the point where they fry... It'd probably work on multiple levels before turning into a smokey mess or combusting. But even then, frying really isn't the same thing as toasting them.
You want a dry frying pan on a hob, or a grill.
Honestly, I had never heard of the method as toasting is not really a function of a microwave, but looking it up it is actually a listed method which can easily be found via searching. It would not be my personal choice, but should actually work as butter/oil is used and the microwave will heat the oil/butter and effectively create a frying environment. Personal opinion is that the results will be different, and to me inferior, to pan or oven results, but if you are happy with the microwave results, that is all that matters to you, not other people's preference.
Microwave prejudice aside, I think you will find pretty much all instructions, microwave, stove top or oven for home toasting will tend to call for single layer for best results. When you start layering you will start having less consistent heating in a pan or on a tray and increase the chances of under toasting, burning, etc. On a stove top you at least have the ability to continually stir to reduce this, but not so much in an oven or a microwave. A microwave is also very limited in its ability to evenly heat thick or layered objects. It heats by vibrating water and some similar shapes, and layering can cause it to not be able to do this in all regions evenly while a single layer is much more likely to give acceptable results.
One more aspect, this is a application which, especially while experimenting is likely to be error prone. When experimenting, until you know the results and if they are what you are looking for, to you want to risk ruining a large batch, or a small one? I would suggest starting small, and if you like the results then consider increasing batch sizes. With all such experiments though, expect more than a few failures, especially when attempting to violate stated recipes. Sometimes recipe statements are just tradition, but often they are also from people who have gone through the failure process and are trying to save you from the same fate. In this instance, I think they are trying to save you the failures.
Ultimately, what is the effect you are looking for?
Toasting is generally used to alter the color and texture of nuts, making them crunchier and imparting a , well 'toasty' flavor. Browning = maillard reaction.
Warming nuts loosens the texture by heating the high oil content of nuts. This will make it easier to puree, etc..
Microwaving will not brown, or 'toast' the nuts until extreme heat is reached from the inside.
Toasting occurs on the surface, and that is why it is desirable to do in a single layer, because only the surface exposed to the hot air or pan will get the surface heating that is desirable.
If you must use a microwave to heat these peanuts, you must heat for a bit, then stir to ensure even heating.