Multi Math

Multi Math is found (after standard installation) in the shader editor properties tab, and look like this:

After you press new to add your first math node to the node group, more options becomes available:

When the node in the list is highlighted (as in the picture above), you can set the type (Add, subtract, Multiply etc.) and the value you want to perform that operation with to your first math node in the series. After that, you just press new for as many nodes you want to have in series in your node group, highlight them and set the type and value. You can also remove any nodes and change the order of them with  UP and DOWN. The next step is to chose how many sockets for the group node you need in Parallel operations. For example, you might have two or more different tasks that needs to do the same mathematical operations separate of each other. This option lets you connect them to the same node.

Once you are finished with your node setup, just press Create node group and your node group is found under the Group section where you add your shader nodes and is called Multimath. You can create as many and different node groups as you like, they will just follow conventional Blender naming, Multimath.001 etc.

Limitations

Multi Math has some limitations. The most important one is that it currently only works in the Shader editor. The second is that you cannot combine different node sockets, for example if you want to combine 10 sockets to one with for example Add or  Multiply, you still need to do it with several math nodes combining two at a time.