Can multi-project wafer service merge projects with different number of layers?
If i need for example 30 layers for my design, and there are only projects with demand of 15..20 layers

It does not work exactly like that.

You talk about layers but what you mean is masks. Not every layer in your design directly relates to a mask. Some AND/OR/NOR... functions are used to translate the (design) layers into physical masks.

For a given IC manufacturing process a certain minimum number of masks is needed. You're not "free" to select which layers/masks you want to use as the IC manufacturing process is required to follow a certain fixed "recipe" to make the ICs. Only some deviations are allowed, meaning some masks are optional, for example a mask for a high-resistance poly-resistor or a mask which defines an extra metal layer.

On a Multi-project-wafer all masks needed by all projects must be present. If there are 10 different designs and only one needs the high-res poly mask then it will actually be available for all those projects. The masks are shared for the MPW so you have no choice.

You do have the choice to use the mask option or not. If you don't use the high-res poly resistor in your design you are free to do so. This also means that if your design goes into mass production you do not need that high-res poly mask. So then you will not be paying for it.

If none of the designs on the MPW use the optional mask then in principle it could be skipped for the MPW as well.

Documents about this are generally not available to anyone unless you're an employee or customer at some Foundry. My answer is based on my personal experience as an IC designer and dealing with these foundries.