1 time.....we can only subtract 1 time because if we subtract two from 50 the answer will be 48 remember, the question is how many times we can subtract 2 from 50....... 
1. it is 1 time only
2. it is because if you subtract 2 from 50 you will get you answer as 48 so it is only 1 time

hope so my answer is useful...^_^ !
Why only once?
You can subtract as many times as you want.
when 2 is subtracted  the answer is 48 {remember the question} it was askedHow many times can you subtract the number two from the number 50 not to subtract from 48