Here's an example juxtaposing a standard <select> element with one upgraded by the mdl-number-select class:
Here's a standard <select> element. It will behave normally when activated.
Here's another element. Its opening tag looks like this: <select class="mdl-number-select">. Because it has the class applied to it, when activated, it will automatically transform the way the input interaction works.
Read the usage section of the project's readme for more information on how to implement the component.