How to use the restrictive clause correct?

Hi~ Everyone here!
I have a question about the line between the use of the non-restrictive clauses and restrictive clauses~
If I say yesterday I wrote a letter to my mom, in which (or in which, with no comma) I detailed what happened to my older brother.
Which one is correct?
Does the clause with no comma imply that I wrote at least two letters that day, or ??????
If you know how to distinguish them correctly, please kindly tell me, many thanks.
I have read many grammar books and on-line info about it, but sometimes I am still confused with some sentences in news articles.