You can do either. 

For a number field, just set the max value to 1,000,000,000. That way only 9 digit numbers can be put in. You could set the minimum value to 99,999,999 which would restrict any 8 or lower digit numbers as well. 

For a single line field, you can use a regular expression like \d{9} to denote that only 9 digit inputs are accepted

If this 9 digit number is actually a number (like a cost of something), it's better to make it a number field. That way in reporting you can add them up, find averages, do other things related to it being an actual number. If this 9 digit number is more like an ID number, it's probably better to treat it like text in a single line. You would never need to add up or average out ID numbers.