If you didn’t know, then it’s excusable… but now you know!
Generally speaking, you don’t have to give a specific number either. A general inclination is good enough for most things. For example, a person struggling with masturbation might confess that they are guilty of such, and the implication is that the number is “lots”. So if there’s a number implied by the confession of the sin itself, one doesn’t necessarily need to make an overt statement of number. Here there is a possibility that the person actually committed the sin only a few times or once, and if that is the case, then they MIGHT make a quantitative statement so that the priest understands that they are NOT guilty of repeating the act a multitude of times.
Where it becomes more of an issue is with sins that are more rare… such as the previous poster’s example of murder. If you murdered 3 people, that’s grossly different than simply saying you murdered (which would seem to imply one murder). So in cases where defining a specific number gives a more clear picture of scope, a quantitative statement SHOULD be made.