As far as I know forgotten mortal sins that are later recalled should be mentioned in the next confession. So, I would think if you confessed “I committed X (mortal sin) once” and later realized you did it two or three times or whatever, you would mention the other times at your next confession.
I don’t believe you’re expected to have a vivid recollection of every single time you committed a certain sin. This becomes particularly impractical when the sin is habitual. So if you were to confess, say, dishonouring your parents, and said you did it “about 50 times” because that was your best estimate and then later recalled a certain specific time you committed that sin, I don’t think you would have to mention it again unless you realized that the number of times you confessed to is inaccurate. Because it could be the case that someone committed some mortal sin hundreds of times, it would be morally impossible to recall specifically each one of those.