Microsoft Forms (Require Number in Text Field)

There is now a way to do this - when you create the question or edit it, there is a set of three dots (...) at the bottom right corner of the question box (next to the "required" option), click on that and select "restrictions". This will give you a set of drop-down menus to select restrictions on the text than can be added by the person completing the form. The default restriction class is "number" and option "is number", which does what you want.

You can do other things like specify a maximum value for the number or a range, you can also specify the class to be text, email, URL or even put a limit on the length