If you insert a Legacy Text Form Field where you want the number
You can then go to the Properties dialog for the FormField and set the type as Number and then select the formatting for the number that you want
FOR THAT TO WORK HOWEVER, THE DOCUMENT MUST BE PROTECTED FOR FILLING IN FORMS, which will severely restrict other things that you can do with the document.
Alternatively, if you insert Content Controls where you want to enter the numbers and you set the Title of each of those Content Controls to "Number", if you have the following code in the ThisDocument object for the document, (or of the Template from which the document is created), the desired formatting will be applied if a number is entered into the Content Control
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim CC As ContentControl
With ActiveDocument
For Each CC In .ContentControls
If CC.Title = "Number" And IsNumeric(CC.Range.Text) Then
CC.Range.Text = Format(CC.Range.Text, "#,##0.00")
End If
Next CC
End With
End Sub
Either the document, or the template from which it is created will need to be saved in Macro enabled format.