There are a few ways you could do this. You need a calculation and then a method to trigger it.
For the calculation you might use:
1. If you know the number you want is always the last 'word' in the field, you can use:
RightWords ( SourceField ; 1 )
2. If you know the number will always be a number, the only number, and not start with zero, you can use:
GetAsNumber ( SourceField )
For the trigger, you could use:
1. A button that runs a single step: Set Field [ TargetField; calculation ]
2. Auto-enter calculation on the target field which has "Do not replace existing value" unchecked
3. A script trigger on the source field if data is manually entered
There may also be other ways. Would one of these work for you?