You’ve been reading too many blogs,

Good call!
Wait we have to say how many times we committed a mortal sin!?
Similar questions to the OP have been coming up in the seven years I’ve been in CAF, that is, where the mortal sins were confessed, but without number, and sometimes at the direction of the priest. The responses usually agree the confession was valid, and then we get into a disagreement as to whether those mortal sins need to be re-confessed with number at another time.
As the discussion progresses, often someone joins in as you have and mentions that they have been confessing without number, unaware that the number is required.
The answer is, as others have mentioned, that yes, the number of mortal sins is required, as a matter of dogma (defined at the Council of Trent, BTW, and also in canon law). Please note that “number” can mean both a specific digit (“once”, “four times”, etc.) or just an honest approximation (“about twice a week”), or even “a few times”, “many times”. If the priest needs more specificity, he will ask. Obviously, “murder” would require a better approximation than sexual habits.
As to whether someone in this situation is now required to revisit previous confessions, then we get into the same disagreement as above.
Unfortunately, the two views on what is to be done about previous confessions (ie. must the omitted numbers now be confessed), has never been answered so well as to give an irrefutable direction.
However, my advise is practical. Once a person is in this quandary about previous confessions then they won’t be satisfied with an answer from the internet anyway, so I recommend they just take the question to the priest next time they are in confession. Priests are always happy to receive such questions - it’s part of the confessional. Put it at the start of the confession, and express it as directly as you can. eg. “In my previous confessions I have confessed mortal sins, but didn’t know that I was required to give the number. Do I have to re-confess those sins?”. Leave it with the priest, and be at peace.
@Cruciferi’s abacus sums up these discussions!

Brilliant!