Well there is no "number lock" because it isn't a traditional keyboard. By choosing &123 in the lower left corner you are essentially number locking by bringing up the number pad and getting rid of the letters. If that isn't quite what you are looking for you can use the split keyboard which gives you all the numbers and the letters. Not sure what other layout you could be asking for.
Even with the split on-screen keyboard, if you tap the '&123' key in the lower left corner, you will get a number pad on the right side of the keyboard until you toggle back by hitting the '&123' key again. Works very well. The number pad in the center of the split on-screen keyboard isn't terrible useful when holding the Surface in your hands, but quickly toggling to numbers keyboard works well.