Translation:I still ask myself how he managed to find my numbers.
Would this be another way of saying: "I still wonder how he managed to find my telephone number" ?
That would be singular, "il mio numero"; unless the speaker has more than one, which is actually rather common.
The only thing that might be strange is numbers being plural. It is a normal sentence in this age of stalkers.
That would simply be 'how he found...'. The question is asking for 'how he managed to find', which is slightly different