Neither. The number is increasing.If the number of somthing was originally 1 and then became 5 and then 7 and now it is 10. Is the number of that thing improving orthe number of that thing is getting improved?
Yes, and certainly nothing has "improved" if the number of mistakes someone makes keeps increasing.... is getting improved suggests that something/someone is doing something to the number (it's an awkward phrase in any case).
See sandpiperlily's explanation as well for the difference between the value of something and the actual number.