I need a way for a user to enter unique numbers and fill those unique
numbers into the empty cells of Sheet2 - column A.

The valid way of entering numbers should be:

123, 124, 125, 126 or
123 124 125 126 or
123-126 meaning all numbers 123 to 126

or a combination of the above. There should be no limitations to how many
numbers that can be entered at once.

Any ideas how to do this? Should I use InputBox or UserForm.